Nonaqueous electrolyte secondary battery

ABSTRACT

Provided is a nonaqueous electrolyte secondary battery, including a positive electrode with a positive electrode active material capable of absorbing and releasing a metal ion; a negative electrode with a negative electrode active material capable of absorbing and releasing a metal ion; and a nonaqueous electrolyte solution; wherein the positive electrode active material includes a lithium transition metal compound, and the positive electrode active material includes at least Ni, Mn and Co, wherein the molar ratio of Mn/(Ni+Mn+Co) is larger than 0 and not larger than 0.32, the molar ratio of Ni/(Ni+Mn+Co) is 0.55 or more, the plate density of the positive electrode is 3.0 g/cm3 or more; and the nonaqueous electrolyte solution includes a monofluorophosphate and/or a difluorophosphate. A total content of the monofluorophosphate and/or difluorophosphate is 0.01% by mass or more in terms of the concentration in the nonaqueous electrolyte solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of prior U.S. applicationSer. No. 16/784,505, filed Feb. 7, 2020; which is a continuation ofInternational Application PCT/JP2018/029618, filed on Aug. 7, 2018, anddesignated the U.S., and claims priority from Japanese PatentApplication 2017-155379 which was filed on Aug. 10, 2017, the entirecontents of which all are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery.

BACKGROUND ART

Since a lithium nonaqueous electrolyte secondary battery using alithium-containing transition metal oxide as a positive electrode, and anonaqueous solvent as an electrolyte solution can achieve high energydensity, it has been applied to a wide range of uses from a small-sizedpower supply for a mobile phone, a laptop computer, or the like, to alarge sized power supply for an automobile or railway, or load leveling.However, in recent year, the demand on a nonaqueous electrolytesecondary battery for higher performance has become more strenuous, andimprovements of various characteristics have been strongly demanded.

For example, Patent Literature 1 describes that a nonaqueous electrolytesecondary battery using an electrolyte solution containing amonofluorophosphate, a difluorophosphate, or the like can exhibit highcapacity, long life, and high output power, even when it is applied to alarge-sized battery.

Patent Literature 2 describes that a positive electrode active materialfor a non-aqueous electrolyte secondary battery having excellent cyclecharacteristics and long life can be stably provided by suppressingdisorder in a crystal of a primary particle of a lithium transitionmetal compound to reduce the resistance of the inside of the crystal,because, when a non-aqueous electrolyte secondary battery is used as apower source for a hybrid vehicle or an electric vehicle, the outputcharacteristics and cycle characteristics are extremely important.

CITATION LIST Patent Literature

-   [Patent Literature 1] International Publication No. WO 2007/055087-   [Patent Literature 2] Japanese Patent Application Laid-Open No.    2007-242288

SUMMARY OF INVENTION Technical Problem

However, despite the recent demand for improvement of thecharacteristics of a nonaqueous electrolyte secondary battery, none ofthe above-mentioned conventional techniques have yet achieved variousperformances of a nonaqueous electrolyte secondary battery all togetherat high levels. For example, with respect to the nonaqueous electrolytesecondary battery of Patent Literature 1, further improvement of thebattery capacity and safety has been required, and with respect to thenonaqueous electrolyte secondary battery of Patent Literature 2, thecapacity retention rate after high temperature storage is low, and theamount of gas from storage, and the dissolution amount of a metal aftera high temperature storage are large, and therefore improvement of thehigh temperature life and enhancement of the safety have been required.In particular, in the case of a large sized battery for an automobile,or the like, the battery itself may be placed in a high temperatureenvironment due to heat from the service environment, such as a heat ofmotor or solar heat. Therefore, a nonaqueous electrolyte secondarybattery, which is excellent in high temperature characteristics under ahigh temperature (for example, the capacity retention rate after hightemperature storage is high, and the amount of gas from storage afterhigh temperature storage is small), the safety is high (for example, theresistance after high temperature storage is low, the amount of metaldissolution from a positive electrode is low, and the amount of heatgeneration at a high temperature is small), has been desired.

An object of the present invention is to provide a nonaqueouselectrolyte secondary battery, in which the capacity retention rateafter high temperature storage is high, the gas amount after hightemperature storage is small, the resistance after high temperaturestorage is low, the amount of metal dissolution from a positiveelectrode is small, and the amount of heat generation at a hightemperature is small.

Solution to Problem

To achieve the object, the present inventors have studied diligently tofind at last that a nonaqueous electrolyte secondary battery in whichthe capacity retention rate after high temperature storage is high, thegas amount after high temperature storage is small, the resistance afterhigh temperature storage is low, the amount of metal dissolution from apositive electrode is small, and the amount of heat generation at a hightemperature is small can be obtained, when the nonaqueous electrolytesecondary battery is produced using a specific positive electrode and anonaqueous electrolyte solution containing a specific compound, therebycompleting the present invention.

That is, the gist of the present invention is as follows.

[1] A nonaqueous electrolyte secondary battery comprising a positiveelectrode with a positive electrode active material capable of absorbingand releasing a metal ion; a negative electrode with a negativeelectrode active material capable of absorbing and releasing a metalion; and a nonaqueous electrolyte solution; wherein

the positive electrode active material comprises a lithium transitionmetal compound, and the positive electrode active material comprises atleast Ni, Mn and Co, wherein a molar ratio of Mn/(Ni+Mn+Co) is largerthan 0 and not larger than 0.32, a molar ratio of Ni/(Ni+Mn+Co) is 0.45or more, the plate density of the positive electrode is 3.0 g/cm³ ormore; and the nonaqueous electrolyte solution comprises amonofluorophosphate and/or a difluorophosphate.

[2] The nonaqueous electrolyte secondary battery according to [1],wherein the positive electrode active material comprises a lithiumtransition metal compound represented by the following Formula (I):Li_(1+x)MO₂  (I)(in the above Formula (I), x is from −0.05 to 0.06, and M comprises atleast Ni, Mn and Co.)[3] The nonaqueous electrolyte secondary battery according to [2],wherein the x is 0.028 or less.[4] The nonaqueous electrolyte secondary battery according to any one of[1] to [3], wherein the molar ratio of Mn/(Ni+Mn+Co) is 0.28 or less.[5] The nonaqueous electrolyte secondary battery according to any one of[1] to [4], wherein the molar ratio of Ni/(Ni+Mn+Co) is 0.55 or more.[6] The nonaqueous electrolyte secondary battery according to any one of[1] to [5], wherein a plate density of the positive electrode is 3.2g/cm³ or more.[7] The nonaqueous electrolyte secondary battery according to any one of[1] to [6], wherein the positive electrode active material furthercontains a sulfate salt.[8] The nonaqueous electrolyte secondary battery according to [7],wherein the amount of the sulfate salt contained in the positiveelectrode active material is 15 μmol/g or more.[9] The nonaqueous electrolyte secondary battery according to any one of[1] to [8], wherein an average Ni valence of the lithium transitionmetal compound is 2.1 or more in an uncharged state.[10] The nonaqueous electrolyte secondary battery according to any oneof [1] to [9], wherein a pH of an aqueous solution of the lithiumtransition metal compound is 11 or higher based on a liquid temperatureof 25° C.[11] The nonaqueous electrolyte secondary battery according to any oneof [1] to [10], wherein the positive electrode active material containsa carbonate salt at 10 μmol/g or more.[12] The nonaqueous electrolyte secondary battery according to any oneof [1] to [11], wherein a tap density of the lithium transition metalcompound is 1.8 g/cm³ or more.

Advantageous Effects of Invention

According to the present invention, a nonaqueous electrolyte secondarybattery, in which the capacity retention rate after high temperaturestorage is high, the gas amount after high temperature storage is small,the resistance after high temperature storage is low, the amount ofmetal dissolution from a positive electrode is small, and the amount ofheat generation at a high temperature is small, can be obtained.

DESCRIPTION OF EMBODIMENTS

An embodiment for implementing the present invention will be describedbelow in detail, provided that the description described below is anexample of embodiments of the present invention (representativeexample), and the present invention is not limited to such contentswithout departing from the gist of the invention as defined in theappended claims.

This embodiment of the present invention relates to a nonaqueouselectrolyte secondary battery provided with a positive electrode with apositive electrode active material capable of absorbing and releasing ametal ion, a negative electrode with a negative electrode activematerial capable of absorbing and releasing a metal ion, and anonaqueous electrolyte solution. Each component will be described below.

[1. Nonaqueous Electrolyte Solution]

A nonaqueous electrolyte solution used in a nonaqueous electrolytesecondary battery of the present invention includes an electrolyte and anonaqueous solvent dissolving it similarly to a general nonaqueouselectrolyte solution, and is mainly characterized in that it contains amonofluorophosphate and/or a difluorophosphate.

[1-1. Monofluorophosphate, and Difluorophosphate]

There is no particular restriction on a monofluorophosphate and adifluorophosphate, insofar as they are respectively salts having atleast one monofluorophosphate structure or difluorophosphate structurein the molecule. When an electrolyte solution containing one or moreselected from a monofluorophosphate and a difluorophosphate is used, thedurability of a nonaqueous electrolyte secondary battery can beimproved. Further, when the electrolyte solution is applied to anonaqueous secondary battery provided with a specific positive electrodedescribed later, a nonaqueous electrolyte secondary battery in which thecapacity retention rate after high temperature storage is high, the gasamount after high temperature storage is small, the resistance afterhigh temperature storage is low, the amount of metal dissolution from apositive electrode is small, and the amount of heat generation at a hightemperature is small can be obtained.

There is no particular restriction on a counter cation for amonofluorophosphate and a difluorophosphate, and examples thereofinclude lithium, sodium, potassium, magnesium, calcium, and an ammoniumrepresented by NR¹²¹R¹²²R¹²³R¹²⁴ (wherein R¹²¹ to R¹²⁴ are independentlyhydrogen or an organic group having 1 to 12 carbon atoms). There is noparticular restriction on the organic group having 1 to 12 carbon atomsrepresented by R¹²¹ to R¹²⁴ of the above ammonium, and examples thereofinclude an alkyl group optionally substituted with a fluorine atom, acycloalkyl group optionally substituted with a halogen atom or an alkylgroup, an aryl group optionally substituted with a halogen atom or analkyl group, and a nitrogen atom-containing heterocyclic groupoptionally having a substituent. Among others, R¹²¹ to R¹²⁴ arepreferably independently a hydrogen atom, an alkyl group, a cycloalkylgroup, or a nitrogen atom-containing heterocyclic group. As a countercation, lithium, sodium, and potassium are preferable, and lithium isparticularly preferable.

Examples of a monofluorophosphate or a difluorophosphate include lithiummonofluorophosphate, sodium monofluorophosphate, potassiummonofluorophosphate, lithium difluorophosphate, sodiumdifluorophosphate, and potassium difluorophosphate, and lithiummonofluorophosphate and lithium difluorophosphate are preferable, andlithium difluorophosphate is more preferable.

The total content of a monofluorophosphate and a difluorophosphate ispreferably 0.01% by mass or more in terms of the concentration in anonaqueous electrolyte solution, more preferably 0.1% by mass or more,especially preferably 0.3% by mass or more, and most preferably 0.5% bymass or more. Meanwhile, it is preferably 8% by mass or less, morepreferably 4% by mass or less, especially preferably 2% by mass or less,and most preferably 1.5% by mass or less. When the total content of amonofluorophosphate and a difluorophosphate is within the range, and anonaqueous electrolyte secondary battery is produced therewith, thepost-storage capacity can be large, and battery swelling or the amountof metal dissolution can be suppressed, so that it can be superior inhigh temperature life and safety, and increase in production cost of anonaqueous electrolyte secondary battery can be avoided.

The monofluorophosphate and difluorophosphate may be used singly, or inan optional combination of two or more kinds thereof at an optionalratio.

In the present invention, a monofluorophosphate or a difluorophosphateincludes that produced in an electrolyte solution or in a battery.

[1-2. Electrolyte]

There is no particular restriction on an electrolyte to be used in anonaqueous electrolyte solution, and any publicly known one can be used,insofar as it is usable in a nonaqueous electrolyte secondary battery asan electrolyte. When a nonaqueous electrolyte solution is used for alithium secondary battery, a lithium salt is usually used as anelectrolyte.

Specific example of an electrolyte include an inorganic lithium salt,such as LiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiSbF₆, LiSO₃F, and LiN(FSO₂)₂; afluorine-containing organic lithium salt, such as LiCF₃SO₃,LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, cyclic lithiumhexafluoropropane-1,3-disulfonylimide, cyclic lithiumtetrafluoroethane-1,2-disulfonylimide, LiN(CF₃SO₂)(C₄F₉SO₂),LiC(CF₃SO₂)₃, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(CF₃SO₂)₂,LiPF₄(C₂F₅SO₂)₂, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂, LiBF₂(CF₃SO₂)₂, andLiBF₂(C₂F₅SO₂)₂; and a lithium salt of a dicarboxylic acid-containingcomplex, such as lithium bis(oxalato)borate, lithiumdifluorooxalatoborate, lithium tris(oxalato)phosphate, lithiumdifluorobis(oxalato)phosphate, and lithiumtetrafluoro(oxalato)phosphate.

Among these, LiPF₆, LiBF₄, LiSO₃F, LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂),LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium bis(oxalato)borate, lithiumdifluoro(oxalato)borate, lithium tris(oxalato)phosphate, lithiumdifluorobis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphateare preferable from the viewpoints of solubility and dissociation degreein a nonaqueous solvent, electrical conductivity, and characteristics ofan obtained battery; and LiPF₆ and LiBF₄ are particularly preferable.

In the present invention, LiBF₄, LiSO₃F, lithiumdifluoro(oxalato)borate, lithium difluorobis(oxalato)phosphate, orlithium tetrafluoro(oxalato)phosphate includes that produced in anelectrolyte solution or in a battery.

The electrolytes may be used singly, or in an optional combination oftwo or more kinds thereof at an optional ratio. In particular, it ispreferable to use two kinds of specific inorganic lithium salts incombination, or to use an inorganic lithium salt and afluorine-containing organic lithium salt in combination, because gasgeneration at the time of trickle charge is suppressed, or deteriorationduring high temperature storage is suppressed. In particular, it ispreferable to use LiPF₆ and LiBF₄ in combination, or an inorganiclithium salt, such as LiPF₆, or LiBF₄, and a fluorine-containing organiclithium salt, such as LiCF₃SO₃, LiN(CF₃SO₂)₂, or LiN(C₂F₅SO₂)₂ incombination.

Further, when LiPF₆ and LiBF₄ are used in combination, the content ofLiBF₄ with respect to the entire electrolyte is preferably 0.01% by massor more and 50% by mass or less. The content is more preferably 0.05% bymass or more, and particularly preferably 0.1% by mass or more.Meanwhile, the upper limit is more preferably 20% by mass or less,further preferably 10% by mass or less, particularly preferably 5% bymass or less, and most preferably 3% by mass or less. When the ratio isin the above range, a desired effect can be easily obtained, and owingto a low dissociation degree of LiBF₄, increase in the resistance of anelectrolyte solution can be suppressed.

Meanwhile, when an inorganic lithium salt, such as LiPF₆ and LiBF₄; andan inorganic lithium salt, such as LiSO₃F, and LiN(FSO₂)₂; or afluorine-containing organic lithium salt, such as LiCF₃SO₃,LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, cyclic lithiumhexafluoropropane-1,3-disulfonylimide, cyclic lithiumtetrafluoroethane-1,2-disulfonylimide, LiN(CF₃SO₂)(C₄F₉SO₂),LiC(CF₃SO₂)₃, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(CF₃SO₂)₂,LiPF₄(C₂F₅SO₂)₂, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂, LiBF₂ (CF₃SO₂)₂, andLiBF₂(C₂F₅SO₂)₂; or a lithium salt of a dicarboxylic acid-containingcomplex, such as lithium bis(oxalato)borate, lithiumtris(oxalato)phosphate, lithium difluorooxalatoborate, lithiumtris(oxalato)phosphate, lithium difluorobis(oxalato)phosphate, andlithium tetrafluoro(oxalato)phosphate; are used in combination, thepercentage of the inorganic lithium salts in the entire electrolyte isusually 70% by mass or more, preferably 80% by mass or more, and morepreferably 85% by mass or more. Meanwhile, it is usually 99% by mass orless, and preferably 95% by mass or less.

The concentration of an electrolyte in a nonaqueous electrolyte solutionmay be optionally selected insofar as the effect of the presentinvention is not impaired, and it is usually 0.5 mol/L or more,preferably 0.6 mol/L or more, and more preferably 0.8 mol/L or more.Meanwhile, it is usually in a range of 3 mol/L or less, preferably 2mol/L or less, more preferably 1.8 mol/L or less, and further preferably1.6 mol/L or less. When the electrolyte concentration is in the aboverange, the electric conductivity of a nonaqueous electrolyte solution issufficient, and decrease in the electric conductivity, namelydeterioration of the performance of a nonaqueous electrolyte secondarybattery, due to viscosity increase may be suppressed.

[1-3. Nonaqueous Solvent]

As a nonaqueous solvent to be contained in a nonaqueous electrolytesolution, any one of heretofore known solvents of a nonaqueouselectrolyte solution may be selected appropriately.

Examples of a usually used nonaqueous solvent include a cycliccarbonate, an open-chain carbonate, an open-chain or cyclic carboxylicacid ester, an open-chain ether, and a phosphorus-containing organicsolvent, a sulfur-containing organic solvent, and a fluorine-containingaromatic solvent.

Examples of a cyclic carbonate include ethylene carbonate, propylenecarbonate, and butylene carbonate. The carbon number of the cycliccarbonate is usually from 3 to 6. Among the above, ethylene carbonateand propylene carbonate are preferable because an electrolyte dissolveseasily owing to a high dielectric constant, so that the cyclecharacteristics of a nonaqueous electrolyte secondary battery to beconstructed can be favorable.

Examples of an open-chain carbonate include dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methyl n-propyl carbonate, ethyln-propyl carbonate, and di-n-propyl carbonate. The carbon number of analkyl group constituting the open-chain carbonate is preferably from 1to 5, and particularly preferably from 1 to 4. Among others, dimethylcarbonate, diethyl carbonate, and ethyl methyl carbonate are preferablefrom the viewpoint of improving battery characteristics.

Also, an open-chain carbonate in which part of hydrogens of the alkylgroup is substituted with fluorine may be included. Examples of theopen-chain carbonate substituted with fluorine include bis(fluoromethyl)carbonate, bis(difluoromethyl) carbonate, bis(trifluoromethyl)carbonate, bis(2-fluoroethyl) carbonate, bis(2,2-difluoroethyl)carbonate, bis(2,2,2-trifluoroethyl) carbonate, 2-fluoroethyl methylcarbonate, 2,2-difluoroethyl methyl carbonate, and 2,2,2-trifluoroethylmethyl carbonate.

Examples of the open-chain carboxylic ester include methyl acetate,ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate,sec-butyl acetate, isobutyl acetate, t-butyl acetate, methyl propionate,ethyl propionate, propyl propionate, isopropyl propionate, methylbutyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethylisobutyrate, methyl valerate, ethyl valerate, methyl pivalate, and ethylpivalate, as well as an open-chain carboxylic ester which is obtained bysubstituting part of hydrogens of any of the afore-listed compounds withfluorine. Examples of such an open-chain carboxylic ester substitutedwith fluorine include methyl trifluoroacetate, ethyl trifluoroacetate,propyl trifluoroacetate, butyl trifluoroacetate, and2,2,2-trifluoroethyl trifluoroacetate.

Among these, methyl acetate, ethyl acetate, propyl acetate, butylacetate, methyl propionate, ethyl propionate, propyl propionate, methylbutyrate, ethyl butyrate, methyl valerate, methyl isobutyrate, ethylisobutyrate, and methyl pivalate are preferable from the viewpoint ofimprovement of battery characteristics.

Examples of the cyclic carboxylic acid ester include γ-butyrolactone,γ-valerolactone, and a cyclic carboxylic acid ester, which is obtainedby substituting part of hydrogens of any of the above compounds withfluorine.

Among these, γ-butyrolactone is more preferable.

Examples of the open-chain ether includes dimethoxymethane,1,1-dimethoxyethane, 1,2-dimethoxyethane, diethoxymethane,1,1-diethoxyethane, 1,2-diethoxyethane, ethoxymethoxymethane,1,1-ethoxymethoxyethane, 1,2-ethoxymethoxyethane, and an open-chainether, which is obtained by substituting part of hydrogens of theafore-listed compound with fluorine.

Examples of such an open-chain ether substituted with fluorine includebis(trifluoroethoxy)ethane, ethoxytrifluoroethoxyethane,methoxytrifluoroethoxyethane,1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethyl-pentane,1,1,1,2,2,3,4,5,5,5-decafluoro-3-ethoxy-4-trifluoromethyl-pentane,1,1,1,2,2,3,4,5,5,5-decafluoro-3-propoxy-4-trifluoromethyl-pentane,1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and2,2-difluoroethyl-2,2,3,3-tetrafluoropropyl ether.

Among these, 1,2-dimethoxyethane, and 1,2-diethoxyethane are morepreferable.

Examples of the phosphorus-containing organic solvent include trimethylphosphate, triethyl phosphate, dimethyl ethyl phosphate, methyl diethylphosphate, ethylene methyl phosphate, ethylene ethyl phosphate,triphenyl phosphate, trimethyl phosphite, triethyl phosphite, triphenylphosphite, trimethylphosphine oxide, triethylphosphine oxide, andtriphenylphosphine oxide, as well as a phosphorus-containing organicsolvent, which is obtained by substituting part of hydrogens of any ofthe afore-listed compounds with fluorine. Examples of such aphosphorus-containing organic solvent substituted with fluorine includetris(2,2,2-trifluoroethyl) phosphate, andtris(2,2,3,3,3-pentafluoropropyl) phosphate.

Examples of the sulfur-containing organic solvent include sulfolane,2-methylsulfolane, 3-methylsulfolane, dimethyl sulfone, diethyl sulfone,ethyl methyl sulfone, methyl propyl sulfone, dimethyl sulfoxide, methylmethanesulfonate, ethyl methanesulfonate, methyl ethanesulfonate, ethylethanesulfonate, dimethyl sulfate, diethyl sulfate, and dibutyl sulfate,as well as a sulfur-containing organic solvent, which is obtained bysubstituting part of hydrogens of any of the afore-listed compounds withfluorine.

Examples of the fluorine-containing aromatic solvent includefluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene,pentafluorobenzene, hexafluorobenzene, and benzotrifluoride.

Among the above nonaqueous solvents, it is preferable to use ethylenecarbonate and/or propylene carbonate, which is a cyclic carbonate, andalso to use any of the above together with an open-chain carbonate fromthe viewpoint of achieving both high conductivity and low viscosity ofthe electrolyte solution.

The nonaqueous solvents may be used singly, or in an optionalcombination of two or more kinds thereof at an optional ratio. In a casewhere two or more kinds are used in combination, for example, when acyclic carbonate and an open-chain carbonate are used in combination, apreferred content of the open-chain carbonate in a nonaqueous solvent isusually 20% by volume or more, and preferably 40% by volume or more,however usually 95% by volume or less, and preferably 90% by volume orless. Meanwhile, a preferred content of the cyclic carbonate in anonaqueous solvent is usually 5% by volume or more, and preferably 10%by volume or more, however usually 80% by volume or less, and preferably60% by volume or less. When the content of the open-chain carbonate isin the above range, increase in the viscosity of a nonaqueouselectrolyte solution may be suppressed, and decrease in the electricconductivity of a nonaqueous electrolyte solution due to decrease in thedissociation degree of a lithium salt serving as an electrolyte may besuppressed. Although the volume of a nonaqueous solvent is hereinmeasured at 25° C., but when the solvent is solid at 25° C. as in thecase of ethylene carbonate, the volume is measured at the melting point.

[1-4. Other Additives]

Various additives may be included to the extent the effect of thepresent invention be not significantly impaired. As the additives, thoseheretofore publicly known may be arbitrarily used. In this regard, theadditives may be used singly, or in an optional combination of two ormore kinds thereof at an optional ratio.

Examples of a heretofore publicly known additive which may be added to anonaqueous electrolyte solution include a cyclic carbonates having acarbon-carbon unsaturated bond, a fluorine-containing cyclic carbonates,a compound having an isocyanate group, a sulfur-containing organiccompound, a phosphorus-containing organic compound, an organic compoundhaving a cyano group, a silicon-containing compound, an aromaticcompound, a fluorine-free carboxylic acid ester, a cyclic compoundhaving a plurality of ether bonds, a compound having the skeleton ofisocyanuric acid, a borate, an oxalate, and a fluorosulfonate.

Hereinafter, each additive will be described, however some of them mayhave be already referred to above.

[1-4-1. Cyclic Carbonate Having Carbon-Carbon Unsaturated Bond]

There is no particular restriction on the cyclic carbonate having acarbon-carbon unsaturated bond (hereinafter sometimes referred to as“unsaturated cyclic carbonate”), insofar as it is a cyclic carbonatehaving a carbon-carbon double bond, or a carbon-carbon triple bond, andany unsaturated carbonate may be used. In this regard, a cycliccarbonate having an aromatic ring is deemed to be included in theunsaturated cyclic carbonate.

Examples of the unsaturated cyclic carbonate include a vinylenecarbonate, an ethylene carbonate substituted with a substituent havingan aromatic ring, a carbon-carbon double bond, or a carbon-carbon triplebond, a phenyl carbonate, a vinyl carbonate, an allyl carbonate, and acatechol carbonate.

Among others, examples of an unsaturated cyclic carbonate that isparticularly favorable for use in combination include vinylenecarbonate, methylvinylene carbonate, 4,5-dimethylvinylene carbonate,vinylvinylene carbonate, 4,5-divinylvinylene carbonate, allylvinylenecarbonate, 4,5-diallylvinylene carbonate, vinylethylene carbonate,4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate,4-methyl-5-allylethylene carbonate, 4-allyl-5-vinylethylene carbonate,ethynylethylene carbonate, 4,5-diethynylethylene carbonate,4-methyl-5-ethynylethylene carbonate, and 4-vinyl-5-ethynylethylenecarbonate. Vinylene carbonate, vinylethylene carbonate, andethynylethylene carbonate are preferable, because they form a morestable interface protective film, vinylene carbonate, and vinylethylenecarbonate are more preferable, and vinylene carbonate is furtherpreferable.

There is no particular restriction on the molecular weight of theunsaturated cyclic carbonate, and it may be arbitrarily selected to theextent that the effect of the present invention be not significantlyimpaired. The molecular weight is preferably 80 or more, and morepreferably 85 or more, however preferably 250 or less, and morepreferably 150 or less. Within the range, it is easy to secure thesolubility of the unsaturated cyclic carbonate in a nonaqueouselectrolyte solution, and to obtain sufficiently the effect of thepresent invention.

There is no particular restriction on the method for producing anunsaturated cyclic carbonate, and unsaturated cyclic carbonates can beproduced by an optional publicly known method.

The unsaturated cyclic carbonates may be used singly, or in an optionalcombination of two or more kinds thereof at an optional ratio. Further,there is no particular restriction on the blending amount of anunsaturated cyclic carbonate, and is arbitrary to the extent that theeffect of the present invention is not significantly impaired. Theblending amount of an unsaturated cyclic carbonate may be 0.001% by massor more in a nonaqueous electrolyte solution as 100% by mass, preferably0.01% by mass or more, more preferably 0.1% by mass or more, and furtherpreferably 0.5 mass % or more, however may be 10% by mass or less,preferably 5% by mass or less, more preferably 4% by mass or less,further preferably 3% by mass or less, and particularly preferably 2% bymass or less %. Within the range, an adequate improving effect on thecycle characteristics of a nonaqueous electrolyte secondary battery islikely to be obtained, and further the high temperature storagecharacteristics can be superior, the gas generation amount can be small,and the discharge capacity retention rate can be superior.

[1-4-2. Fluorine-Containing Cyclic Carbonate]

Examples of the fluorine-containing cyclic carbonate include afluorinated product of a cyclic carbonate having an alkylene groupusually having a carbon number of 2 to 6, and a derivative thereof, suchas a fluorinated product of ethylene carbonate (hereinafter sometimesreferred to as “fluorinated ethylene carbonate”), and a derivativethereof. Examples of a derivative of a fluorinated product of ethylenecarbonate include a fluorinated product of ethylene carbonatesubstituted with an alkyl group having a carbon number of 1 to 4. Amongothers, a fluorinated ethylene carbonate having a fluorine number of 1to 8 and a derivative thereof are preferable.

By adding a fluorine-containing cyclic carbonate to an electrolytesolution, the high temperature storage characteristics and cyclecharacteristics of a battery using the electrolyte solution can beimproved.

Examples of a fluorinated ethylene carbonate having a fluorine number of1 to 8 and a derivative thereof include monofluoroethylene carbonate,4,4-difluoroethylene carbonate, 4,5-difluoroethylene carbonate,4-fluoro-4-methyl ethylene carbonate, 4,5-difluoro-4-methyl ethylenecarbonate, 4-fluoro-5-methyl ethylene carbonate, 4,4-difluoro-5-methylethylene carbonate, 4-(fluoromethyl)ethylene carbonate,4-(difluoromethyl)ethylene carbonate, 4-(trifluoromethyl)ethylenecarbonate, 4-(fluoromethyl) 4-fluoroethylene carbonate, 4-(fluoromethyl)5-fluoroethylene carbonate, 4-fluoro-4,5-dimethyl ethylene carbonate,4,5-difluoro-4,5-dimethyl ethylene carbonate, and4,4-difluoro-5,5-dimethyl ethylene carbonate.

Among them, monofluoroethylene carbonate, 4,4-difluoroethylenecarbonate, and 4,5-difluoroethylene carbonate are preferable, becausethey give high electrolytic conductivity to an electrolyte solution, andcan easily form a stable interface protective film.

The fluorinated cyclic carbonates may be used singly, or in an optionalcombination of two or more kinds thereof at an optional ratio. Theamount of a fluorinated cyclic carbonate (total amount in the case of 2or more kinds) in an electrolyte solution as 100% by mass is preferably0.001% by mass or more, more preferably 0.01% by mass or more, furtherpreferably 0.1% by mass or more, still further preferably 0.5% by massor more, particularly preferably 1% by mass or more, and most preferably1.2% by mass or more, meanwhile is preferably 10% by mass or less, morepreferably 7% by mass, further preferably 5% by mass or less,particularly preferably 3% by mass or less, and most preferably 2% bymass or less. Further, when the fluorinated cyclic carbonate is used asa nonaqueous solvent, the blending amount in a nonaqueous solvent as100% by volume is preferably 1% by volume or more, more preferably 5% byvolume or more, and further preferably 10% by volume or more, meanwhileis preferably 50% by volume or less, more preferably 35% by volume orless, and further preferably 25% by volume or less.

By using the carbonate at the aforedescribed content, it becomespossible to obtain sufficient improving effects on high temperaturestorage characteristics and cycle characteristics, and also to suppressunnecessary gas generation.

[1-4-3. Compound Having Isocyanate Group]

A nonaqueous electrolyte solution may contain a compound having anisocyanate group. Hereinafter, it may be sometimes referred to as“isocyanate compound”.

There is no particular restriction on the isocyanate compound, insofaras it is an organic compound having at least one isocyanate group in themolecule, however the number of isocyanate groups is preferably from 1to 4 in the molecule, more preferably from 1 to 3, and furtherpreferably 1 or 2.

The isocyanate compound is preferably a compound in which an isocyanategroup is bonded with: an straight chain or branched alkylene group, acycloalkylene group, a structure in which a cycloalkylene group and analkylene group are linked together, an aromatic hydrocarbon group, astructure in which an aromatic hydrocarbon group and an alkylene groupare linked together, an ether structure (—O—), a structure in which anether structure (—O—) and an alkylene group are linked together, acarbonyl group (—C(═O)—), a structure in which a carbonyl group and analkylene group are linked together, a sulfonyl group (—S(═O)—), astructure in which a sulfonyl group and an alkylene group are linkedtogether, or a compound having, for example, a structure where one ofthe above is halogenated; more preferably a compound in which anisocyanate group is bonded with: an straight chain or branched alkylenegroup, a cycloalkylene group, a structure in which a cycloalkylene groupand an alkylene group are linked together, or an aromatic hydrocarbongroup or a structure in which an aromatic hydrocarbon group and analkylene group are linked together; and further preferably a compound inwhich an isocyanate group is bonded with a structure in which acycloalkylene group and an alkylene group are linked together. There isno particular restriction on the molecular weight of the isocyanatecompound. The molecular weight is preferably 80 or more, more preferably115 or more, and further preferably 170 or more, however is 300 or less,and more preferably 230 or less. Within the range, it is easy to securethe solubility of the isocyanate compound in a nonaqueous electrolytesolution, so that the effect of the present invention is easilyobtained. There is no particular restriction on a method for producingthe isocyanate compound, and it can be produced by arbitrarily selectinga publicly known method. Alternatively, a commercial product may beused.

Examples of the isocyanate compound include:

as a compound having one isocyanate group, an alkyl isocyanate, such asmethyl isocyanate, ethyl isocyanate, propyl isocyanate, isopropylisocyanate, butyl isocyanate, and tert-butyl isocyanate; a cycloalkylisocyanate, such as cyclohexyl isocyanate, and an unsaturatedisocyanate, such as allyl isocyanate and propargyl isocyanate; and anaromatic isocyanate, such as phenyl isocyanate, trifluoromethylphenylisocyanate, and p-toluenesulfonyl isocyanate;

As a compound having two isocyanate groups, a compound, such asmonomethylene diisocyanate, dimethylene diisocyanate, tetramethylenediisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate,decamethylene diisocyanate, 1,4-diisocyanato-2-butene, toluenediisocyanate, xylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane,1,4-diisocyanatocyclohexane, dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methyl isocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methyl isocyanate), isophoronediisocyanate, carbonyl diisocyanate, 1,4-diisocyanatobutane-1,4-dione,and trimethylhexamethylene diisocyanate;

As a compound having three isocyanate groups, a compound, such as1,6,11-triisocyanatoundecane, 4-isocyanatomethyl-1,8-octamethylenediisocyanate, 1,3,5-triisocyanate methylbenzene,1,3,5-tris(6-isocyanatohexan-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,and a trimer compound derived from a compound having at least twoisocyanate groups in the molecule (e.g. biuret, an isocyanurate, anadduct and a bifunctional type modified polyisocyanate).

Among these, compounds, such as t-butyl isocyanate, cyclohexylisocyanate, p-toluenesulfonyl isocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, decamethylene diisocyanate,1,3-bis(isocyanatomethyl)cyclohexane,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methylisocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methylisocyanate), isophoronediisocyanate, and trimethylhexamethylene diisocyanate are preferablefrom the viewpoint of improvement of the storage characteristics;cyclohexyl isocyanate, p-toluenesulfonyl isocyanate, hexamethylenediisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methylisocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methyl isocyanate), isophoronediisocyanate, and trimethylhexamethylene diisocyanate are morepreferable; and cyclohexyl isocyanate, p-toluenesulfonyl isocyanate,hexamethylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane,dicyclohexylmethane-4,4′-diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methylisocyanate), andbicyclo[2.2.1]heptane-2,6-diylbis(methylisocyanate) are furtherpreferable.

The isocyanate compounds may be used singly, or may be included in anoptional combination of two or more kinds thereof at an optional ratio.

The amount of the isocyanate compound (total amount in the case of twoor more kinds) in an electrolyte solution as 100% by mass may be 0.001%by mass or more, and is preferably 0.1% by mass or more, and morepreferably 0.3% by mass or more, and may be 10% by mass or less, and ispreferably 5% by mass or less, and more preferably 3% by mass or less.If the amount is within the above range, the output characteristics,load characteristics, low temperature characteristics, cyclecharacteristics, high temperature storage characteristics, and othercharacteristics can be easily controlled.

[1-4-4. Sulfur-Containing Organic Compound]

Although there is no particular restriction on the sulfur-containingorganic compound insofar as it is an organic compound having at leastone sulfur atom in the molecule, it is preferably an organic compoundhaving an S═O group in the molecule. Examples thereof include anopen-chain sulfonate, a cyclic sulfonate, an open-chain sulfate, acyclic sulfate, an open-chain sulfite, and a cyclic sulfite. However,that corresponding to a fluorosulfonate is not regarded as asulfur-containing organic compound described later, rather it isincluded in the fluorosulfonate serving as an electrolyte describedlater.

Among these, an open-chain sulfonate, a cyclic sulfonate, an open-chainsulfate, a cyclic sulfate, an open-chain sulfite, and a cyclic sulfiteare preferable, and a compound having an S(═O)₂ group is morepreferable.

More preferable are an open-chain sulfonate and a cyclic s sulfonate,and a cyclic sulfonate is further preferable. Specific examples ofcompounds of an open-chain sulfonate, a cyclic sulfonate, an open-chainsulfate, a cyclic sulfate, an open-chain sulfite, and a cyclic sulfiteinclude the following.

<Open-Chain Sulfonate>

A fluorosulfonate, such as methyl fluorosulfonate, and ethylfluorosulfonate.

A methanesulfonate, such as methyl methanesulfonate, ethylmethanesulfonate, busulfan, methyl 2-(methanesulfonyloxy)propionate,ethyl 2-(methanesulfonyloxy)propionate, and ethylmethanesulfonyloxyacetate.

An alkenyl sulfonate, such as methyl vinyl sulfonate, ethyl vinylsulfonate, allyl vinyl sulfonate, propargyl vinyl sulfonate, methylallyl sulfonate, ethyl allyl sulfonate, allyl allyl sulfonate, propargylallyl sulfonate, and 1,2-bis(vinylsulfonyloxy)ethane.

An alkyl disulfonate, such as methoxycarbonylmethyl methanedisulfonate,ethoxycarbonylmethyl methanedisulfonate, methoxycarbonylmethyl1,2-ethanedisulfonate, ethoxycarbonylmethyl 1,2-ethanedisulfonate,methoxycarbonylmethyl 1,3-propanedisulfonate, ethoxycarbonylmethyl1,3-propanedisulfonate, and 1-methoxycarbonylethyl1,3-propanedisulfonate.

<Cyclic Sulfonate>

A sultone compound, such as 1,3-propane sultone, 1-fluoro-1,3-propanesultone, 2-fluoro-1,3-propane sultone, 3-fluoro-1,3-propane sultone,1-methyl-1,3-propane sultone, 2-methyl-1,3-propane sultone,3-methyl-1,3-propane sultone, 1-propene-1,3-sultone,2-propene-1,3-sultone, 2-methyl-1-propene-1,3-sultone, 1,4-butanesultone, and 1,5-pentane sultone.

A disulfonate compound, such as methylene methanedisulfonate andethylene methanedisulfonate.

A nitrogen-containing compound, such as1,2,3-oxathiazolidine-2,2-dioxide.

A phosphorus-containing compound, such as1,2,3-oxathiaphosphinane-2,2-dioxide.

<Open-Chain Sulfate>

A dialkyl sulfate compound, such as dimethyl sulfate, ethyl methylsulfate, and diethyl sulfate.

<Cyclic Sulfate>

An alkylene sulfate compound, such as 1,2-ethylene sulfate,1,2-propylene sulfate, 1,3-propylene sulfate, 1,2-butylene sulfate,1,3-butylene sulfate, 1,4-butylene sulfate, 1,2-pentylene sulfate,1,3-pentylene sulfate, 1,4-pentylene sulfate, and 1,5-pentylene sulfate.

<Open-Chain Sulfite>

A dialkyl sulfite compound, such as dimethyl sulfite, ethyl methylsulfite and diethyl sulfite.

<Cyclic Sulfite>

An alkylene sulfite compounds such as 1,2-ethylene sulfite,1,2-propylene sulfite, 1,3-propylene sulfite, 1,2-butylene sulfite,1,3-butylene sulfite, 1,4-butylene sulfite, 1,2-pentylenesulfite,1,3-pentylenesulfite, 1,4-pentylenesulfite, and 1,5-pentylenesulfite.

Among these, methyl 2-(methanesulfonyloxy)propionate, ethyl2-(methanesulfonyloxy)propionate, 2-propynyl2-(methanesulfonyloxy)propionate, 1-methoxycarbonylethylpropanedisulfonate, 1-ethoxycarbonylethyl propanedisulfonate,1-methoxycarbonylethyl butanedisulfonate, 1-ethoxycarbonylethylbutanedisulfonate, 1,3-propane sultone, 1-propene-1,3-sultone,1,4-butane sultone, 1.2-ethylene sulfate, 1,2-ethylene sulfite, methylmethanesulfonate, and ethyl methanesulfonate are preferable from theviewpoint of improvement of the initial efficiency;1-methoxycarbonylethyl propanedisulfonate, 1-ethoxycarbonylethylpropanedisulfonate, 1-methoxycarbonylethyl butanedisulfonate,1-ethoxycarbonylethyl butanedisulfonate, 1,3-propane sultone,1-propene-1,3-sultone, 1,2-ethylene sulfate, and 1,2-ethylene sulfiteare more preferable; and 1,3-propane sultone, and 1-propene-1,3-sultoneare further preferable.

The sulfur-containing organic compounds may be used singly, or in anoptional combination of two or more kinds thereof at an optional ratio.

The content of the sulfur-containing organic compound (total amount inthe case of 2 or more kinds) in an electrolyte solution as 100% by massmay be 0.001% by mass or more, is preferably 0.01% by mass or more, morepreferably 0.1% by mass or more, particularly preferably 0.3% by mass ormore, and most preferably 0.6% by mass or more, and may be 10% by massor less, is preferably 5% by mass or less, more preferably 3% by mass orless, further preferably 2% by mass or less, particularly preferably1.5% by mass or less, and most preferably 1.0% by mass or less. If theamount is within the above range, the battery output characteristics,load characteristics, low temperature characteristics, cyclecharacteristics, high temperature storage characteristics, and othercharacteristics can be easily controlled.

[1-4-5. Phosphorus-Containing Organic Compound]

There is no particular restriction on the phosphorus-containing organiccompound, insofar as it is an organic compound having at least onephosphorus atom in the molecule. A battery using a nonaqueouselectrolyte solution containing a phosphorus-containing organic compoundcan improve its durability.

As the phosphorus-containing organic compound, a phosphate, aphosphonate, a phosphinate, and a phosphite are preferable; a phosphate,and a phosphonate are more preferable; and a phosphonate is morepreferable. These esters may have a substituent.

Specific examples of the phosphorus-containing organic compound includediethyl vinyl phosphate, allyl diethyl phosphate, propargyl diethylphosphate, trivinyl phosphate, triallyl phosphate, tripropargylphosphate, diallylethyl phosphate, dipropargylethyl phosphate,2-acryloyloxyethyl diethyl phosphate, tris(2-acryloyloxyethyl)phosphate,trimethyl phosphonoformate, methyl diethylphosphonoformate, methyldipropylphosphonoformate, methyl dibutylphosphonoformate, triethylphosphonoformate, ethyl dimethylphosphonoformate, ethyldipropylphosphonoformate, ethyl dibutylphosphonoformate, tripropylphosphonoformate, propyl dimethylphosphonoformate, propyldiethylphosphonoformate, propyl dibutylphosphonoformate, tributylphosphonoformate, butyl dimethylphosphonoformate, butyldiethylphosphonoformate, butyl dipropylphosphonoformate, methylbis(2,2,2-trifluoroethyl)phosphonoformate, ethylbis(2,2,2-trifluoroethyl)phosphonoformate, propylbis(2,2,2-trifluoroethyl) phosphonoformate, butylbis(2,2,2-trifluoroethyl)phosphonoformate, trimethyl phosphonoacetate,methyl diethylphosphonoacetate, methyl dipropylphosphonoacetate, methyldibutylphosphonoacetate, triethyl phosphonoacetate, ethyldimethylphosphonoacetate, ethyl dipropylphosphonoacetate, ethyldibutylphosphonoacetate, tripropyl phosphonoacetate, propyldimethylphosphonoacetate, propyl diethylphosphonoacetate, propyldibutylphosphonoacetate, tributyl phosphonoacetate, butyldimethylphosphonoacetate, butyl diethylphosphonoacetate, butyldipropylphosphonoacetate, methylbis(2,2,2-trifluoroethyl)phosphonoacetate, ethylbis(2,2,2-trifluoroethyl)phosphonoacetate, propylbis(2,2,2-trifluoroethyl)phosphonoacetate, butylbis(2,2,2-trifluoroethyl)phosphonoacetate, allyldimethylphosphonoacetate, allyl diethylphosphonoacetate, 2-propynyldimethylphosphonoacetate, 2-propynyl diethylphosphonoacetate, trimethyl3-phosphonopropionate, methyl 3-(diethylphosphono)propionate, methyl3-(dipropylphosphono)propionate, methyl 3-(dibutylphosphono)propionate,triethyl 3-phosphonopropionate, ethyl 3-(dimethylphosphono)propionate,ethyl 3-(dipropylphosphono)propionate, ethyl3-(dibutylphosphono)propionate, tripropyl 3-phosphonopropionate, propyl3-(dimethylphosphono) propionate, propyl 3-(diethylphosphono)propionate,propyl 3-(dibutylphosphono)propionate, tributyl 3-phosphonopropionate,butyl 3-(dimethylphosphono)propionate, butyl3-(diethylphosphono)propionate, butyl 3-(dipropylphosphono)propionate,methyl 3-(bis(2,2,2-trifluoroethyl)phosphono)propionate, ethyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate, propyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate, butyl3-(bis(2,2,2-trifluoroethyl)phosphono)propionate, trimethyl4-phosphonobutyrate, methyl 4-(diethylphosphono)butyrate, methyl4-(dipropylphosphono)butyrate, methyl 4-(dibutylphosphono)butyrate,triethyl 4-phosphonobutyrate, ethyl 4-(dimethylphosphono)butyrate, ethyl4-(dipropylphosphono)butyrate, ethyl 4-(dibutylphosphono)butyrate,tripropyl 4-phosphonobutyrate, propyl 4-(dimethylphosphono)butyrate,propyl 4-(diethylphosphono)butyrate, propyl4-(dibutylphosphono)butyrate, tributyl 4-phosphonobutyrate, butyl4-(dimethylphosphono)butyrate, butyl 4-(diethylphosphono)butyrate, andbutyl 4-(dipropylphosphono)butyrate.

The phosphorus-containing organic compounds may be used singly, or in anoptional combination of two or more kinds thereof at an optional ratio.

The content of the phosphorus-containing organic compound (total amountin the case of 2 or more kinds) in an electrolyte solution as 100% bymass may be 0.001% by mass or more, is preferably 0.01% by mass or more,more preferably 0.1% by mass or more, further preferably 0.4% by mass ormore, particularly preferably 0.6% by mass or more, and may be 10% bymass or less, is preferably 5% by mass or less, more preferably 3% bymass or less, further preferably 2% by mass or less, particularlypreferably 1.2% by mass or less, and most preferably 0.9% by mass orless. If the amount is within the above range, the outputcharacteristics, load characteristics, low temperature characteristics,cycle characteristics, high temperature storage characteristics, andother characteristics can be easily controlled.

[1-4-6. Organic Compound Having Cyano Group]

Examples of an organic compound having a cyano group includepentanenitrile, octanenitrile, decanenitrile, dodecanenitrile,crotononitrile, succinonitrile, glutaronitrile, adiponitrile,pimelonitrile, suberonitrile, and3,9-bis(2-cyanoethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane,1,2,3-tricyanopropane, 1,3,5-tricyanopentane, 1,4,7-tricyanoheptane,1,2,4-tricyanobutane, 1,2,5-tricyanopentane, 1,2,6-tricyanohexane,1,3,6-tricyanohexane, and 1,2,7-tricyanoheptane.

The concentration of the organic compound having a cyano group in anelectrolyte solution as 100% by mass may be 0.001% by mass or more, ispreferably 0.01% by mass or more, more preferably 0.1% by mass or more,and particularly preferably 0.3% by mass or more; and may be 10% by massor less, is preferably 5% by mass or less, more preferably 3% by mass orless, and particularly preferably 2% by mass or less. If the amount iswithin the above range, the output characteristics, loadcharacteristics, low temperature characteristics, cycle characteristics,high temperature storage characteristics, and other characteristics canbe easily controlled.

The organic compounds having a cyano group may be used singly, or in anoptional combination of two or more kinds thereof at an optional ratio.

[1-4-7. Silicon-Containing Compound]

There is no particular restriction on the silicon-containing compound,insofar as it is a compound having at least one silicon atom in themolecule. By using an electrolyte solution containing thesilicon-containing compound, the durability of a nonaqueous electrolytesecondary battery can be improved.

As the silicon-containing compound, a compound represented by theFormula (2-6) is preferable.

In Formula (2-6), R⁶¹, R⁶², and R⁶³ are independently a hydrogen atom, ahalogen atom, or a hydrocarbon group having a carbon number of 1 to 10,

X⁶¹ is an organic group including at least one atom selected from thegroup consisting of an oxygen atom, a nitrogen atom, and a silicon atom.

R⁶¹, R⁶², and R⁶³ are preferably a hydrogen atom, a fluorine atom, amethyl group, an ethyl group, a n-propyl group, an isopropyl group, an-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group,and a phenyl group, and more preferably a methyl group.

X⁶¹ is an organic group including at least one atom selected from thegroup consisting of an oxygen atom, a nitrogen atom and a silicon atom,and preferably is an organic group including at least an oxygen atom ora silicon atom. In this regard, the organic group means a group composedof one or more atoms selected from the group consisting of a carbonatom, a hydrogen atom, a nitrogen atom, an oxygen atom, a silicon atom,a sulfur atom, a phosphorus atom, and a halogen atom. Examples of theorganic group include an alkyl group, an alkenyl group, an alkynylgroup, an aryl group, an alkoxy group, a CN group, an isocyanate group,a fluoro group, an alkylsulfonic acid group, and a trialkylsilyl group.Part of monovalent organic groups may be substituted with a fluorineatom. The carbon number of the organic group may be 1 or more,preferably 3 or more, and more preferably 5 or more; and 15 or less,preferably 12 or less, and more preferably 8 or less.

Among these, an alkylsulfonic acid group, a trialkylsilyl group, a boricacid group, a phosphoric acid group, and a phosphorous acid group arepreferable.

Examples of a silicon-containing compound include:

a boric acid compound, such as tris(trimethylsilyl) borate,tris(trimethoxysilyl) borate, tris(triethylsilyl) borate,tris(triethoxysilyl) borate, tris(dimethylvinylsilyl) borate, andtris(diethylvinylsilyl) borate;

a phosphoric acid compound, such as tris(trimethylsilyl)phosphate,tris(triethylsilyl)phosphate, tris(tripropylsilyl)phosphate,tris(triphenylsilyl)phosphate, tris(trimethoxysilyl)phosphate,tris(triethoxysilyl)phosphate, tris(triphenoxysilyl)phosphate,tris(dimethylvinylsilyl)phosphate, and tris(diethylvinylsilyl)phosphate;

a phosphorous acid compound, such as tris(trimethylsilyl)phosphite,tris(triethylsilyl)phosphite, tris(tripropylsilyl)phosphite,tris(triphenylsilyl)phosphite, tris(trimethoxysilyl)phosphite,tris(triethoxysilyl)phosphite, tris(triphenoxysilyl)phosphite,tris(dimethylvinylsilyl)phosphite, and tris(diethylvinylsilyl)phosphite;

a sulfonic acid compound, such as trimethylsilyl methanesulfonate, andtrimethylsilyl tetrafluoromethanesulfonate; and

a disilane compound, such as hexamethyldisilane, hexaethyldisilane,1,1,2,2-tetramethyldisilane, 1,1,2,2-tetraethyldisilane,1,2-diphenyltetramethyldisilane, and 1,1,2,2-tetraphenyldisilane.

Among these, tris(trimethylsilyl) borate, tris(trimethylsilyl)phosphate,tris(trimethylsilyl)phosphite, trimethylsilyl methanesulfonate,trimethylsilyl tetrafluoromethanesulfonate, hexamethyldisilane,hexaethyldisilane, 1,2-diphenyltetramethyldisilane, and1,1,2,2-tetraphenyldisilane are preferable; and tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite,and hexamethyldisilane are more preferable.

The silicon-containing compounds may be used singly, or in an optionalcombination of two or more kinds thereof at an optional ratio.

The amount of the silicon-containing compound (total amount in the caseof 2 or more types) in an electrolyte solution as 100% by mass may be0.001% by mass or more, is preferably 0.1% by mass or more, and morepreferably 0.3% by mass or more; and may be 10% by mass or less, ispreferably 5% by mass or less, and more preferably 3% by mass or less.If the amount is within the range, the output characteristics, loadcharacteristics, low temperature characteristics, cycle characteristics,high temperature storage characteristics, and other characteristics canbe easily controlled.

[1-4-8. Aromatic Compound]

There is no particular restriction on the aromatic compound, insofar asit is an organic compound having at least one aromatic ring in themolecule.

Examples of the aromatic compound include the following:

Fluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene,pentafluorobenzene, hexafluorobenzene, benzotrifluoride,cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, diphenylcarbonate, methylphenyl carbonate, 2-phenylethyl acetate, 3-phenylpropylacetate, methyl phenyl acetate, ethyl phenyl acetate, 2-phenylethylphenyl acetate, 3-phenylpropyl phenyl acetate, methyl3-phenylpropionate, ethyl 3-phenylpropionate, 2-phenylethyl3-phenylpropionate, 3-phenylpropyl 3-phenylpropionate,methylphenylsulfonate, 2-tert-butylphenylmethylsulfonate,4-tert-butylphenylmethylsulfonate, cyclohexylphenylmethylsulfonate,trimethylphenylsilane, tris(2-tert-butylphenyl)phosphate,tris(4-tert-butylphenyl)phosphate, tris(2-cyclohexylphenyl)phosphate,tris(4-cyclohexylphenyl)phosphate, diethylphenylphosphonate,diethylbenzylphosphonate, diethyl-(4-fluorobenzyl)phosphonate,2-fluorophenyl acetate, 4-fluorophenyl acetate, 2,4-difluoroanisole,2-fluorotoluene, 3-fluorotoluene, and 4-fluorotoluene.

Among these, fluorobenzene, benzotrifluoride, cyclohexylbenzene,tert-butylbenzene, tert-amylbenzene, diphenyl carbonate, methylphenylcarbonate, 2-phenylethyl phenyl acetate,4-tert-butylphenylmethylsulfonate, cyclohexylphenylmethylsulfonate,tris(2-tert-butylphenyl)phosphate, tris(4-tert-butylphenyl)phosphate,tris(4-cyclohexylphenyl)phosphate, 2,4-difluoroanisole, and2-fluorotoluene.

Examples include in addition to those listed above:

1-phenyl-1,3,3-trimethyl indan,2,3-dihydro-1,3-dimethyl-1-(2-methyl-2-phenylpropyl)-3-phenyl-1H-indan,2,2-diphenylbutane, 3,3-diphenylpentane, 3,3-diphenylhexane,4,4-diphenylheptane, 5,5-diphenyloctane, 6,6-diphenylnonane,1,1-diphenyl-1,1-di-tert-butyl-methane, 1,1-diphenylcyclohexane,1,1-diphenylcyclopentane, 1,1-diphenyl-4-methylcyclohexane,1,3-bis(1-methyl-1-phenylethyl)-benzene, and1,4-bis(1-methyl-1-phenylethyl)-benzene.

The aromatic compounds may be used singly, or in combination of two ormore kinds thereof. The amount of the aromatic compound (total amount inthe case of 2 or more types) may be 0.001% by mass or more in an entirenonaqueous electrolyte solution as 100% by mass, is preferably 0.01% bymass or more, more preferably 0.05% by mass or more, further preferably0.1% by mass or more, and still further preferably 0.4% by mass or more;and may be 10% by mass or less, is preferably 8% by mass or less, morepreferably 5% by mass or less, further preferably 3% by mass or less,and particularly preferably 2.5% by mass or less. Within the aboverange, the effect of the present invention can be easily expressed, andincrease in the battery resistance can be prevented.

[1-4-9. Fluorine-Free Carboxylic Acid Ester]

The fluorine-free carboxylic acid ester can also be used as a solvent asdescribed above. There is no particular restriction on the fluorine-freecarboxylic acid ester, insofar as it is a carboxylic acid ester havingno fluorine atom in the molecule.

Examples of a fluorine-free open-chain carboxylic acid ester include thefollowing:

methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, methylpropionate, ethyl propionate, n-propyl propionate, n-butyl propionate,methyl butyrate, ethyl butyrate, n-propyl butyrate, n-butyl butyrate,methyl valerate, ethyl valerate, n-propyl valerate, n-butyl valerate,methyl pivalate, ethyl pivalate, n-propyl pivalate, and n-butylpivalate.

Among these, methyl acetate, ethyl acetate, n-propyl acetate, n-butylacetate, methyl propionate, ethyl propionate, n-propyl propionate, andn-butyl propionate are more preferable from the viewpoint of improvingthe ionic conductivity owing to reduction of the viscosity of anelectrolyte solution, methyl propionate, ethyl propionate, n-propylpropionate, and n-butyl propionate are more preferable, and ethylpropionate and n-propyl propionate are particularly preferable.

The fluorine-free carboxylic acid esters may be used singly, or in anoptional combination of two or more kinds thereof at an optional ratio.

The amount of the fluorine-free carboxylic acid ester (total amount inthe case of 2 or more kinds) in an electrolyte solution as 100% by massmay be 0.001% by mass or more, is preferably 0.01% by mass or more, morepreferably 0.1% by mass or more, further preferably 0.3% by mass ormore, and particularly preferably 0.6% by mass or more, and may be 10%by mass or less, is preferably 5% by mass or less, more preferably 3% bymass or less, further preferably 2% by mass or less, and particularlypreferably 1% by mass or less. Further, when the fluorine-freecarboxylic acid ester is used as a nonaqueous solvent, the blendingamount is preferably 1% by volume or more in a nonaqueous solvent as100% by volume of, more preferably 5% by volume or more, furtherpreferably 10% by volume or more, and still further preferably 20% byvolume or more, and may be 50% by volume or less, is more preferably 45%by volume or less, and further preferably 40% by volume or less. Withinthe range, increase in the negative electrode resistance is suppressed,and the output characteristics, load characteristics, low temperaturecharacteristics, cycle characteristics, and high temperature storagecharacteristics can be easily controlled.

[1-4-10. Cyclic Compound Having a Plurality of Ether Bonds]

There is no particular restriction on the cyclic compound having aplurality of ether bonds, insofar as it is a cyclic compound having aplurality of ether bonds in the molecule. The cyclic compounds having aplurality of ether bonds contribute to improvement of the hightemperature storage characteristics of a battery, and is able to improvethe durability of a nonaqueous electrolyte secondary battery.

Examples of the cyclic compound having a plurality of ether bondsinclude tetrahydrofuran, methyltetrahydrofuran, tetrahydropyran, andmethyltetrahydropyran.

The cyclic compounds having a plurality of ether bonds may be usedsingly, or in an optional combination of two or more kinds thereof at anoptional ratio. The amount of the cyclic compound having a plurality ofether bonds (total amount in the case of 2 or more types) in anelectrolyte solution as 100% by mass may be 0.001% by mass or more, ispreferably 0.01% by mass or more, more preferably 0.1% by mass or more,and particularly preferably 0.3% by mass or more, and may be 10% by massor less, is preferably 5% by mass or less, more preferably 3% by mass orless, and further preferably 2% by mass or less. When the amountsatisfies the above range, the output characteristics, loadcharacteristics, low temperature characteristics, cycle characteristics,high temperature storage characteristics, and other characteristics canbe easily controlled.

[1-4-11. Additive that is Electrolyte]

Among the additives, examples of an additive playing a role ofelectrolyte include the following (borate, oxalate, andfluorosulfonate). These salts are particularly preferably in a form of alithium salt.

The total content of a borate, an oxalate and a fluorosulfonate in anonaqueous electrolyte solution is preferably 0.01% by mass or more, andparticularly preferably 0.1% by mass or more. Meanwhile, it ispreferably 20% by mass or less, and particularly preferably 10% by massor less.

[1-4-11-1. Borate]

There is no particular restriction on the borate, insofar as it is asalt having at least one boron atom in the molecule. However, onecorresponding to an oxalate is not regarded as a borate but rather asone of the oxalates described later. The borate is able to improve thedurability of a battery according to the present invention.

Examples of the counter cation of the borate include lithium, sodium,potassium, magnesium, calcium, rubidium, cesium, and barium, among whichlithium is preferable.

As the borate, a lithium salt is preferable, and a fluorine-containinglithium salt can also be suitably used. Examples thereof include LiBF₄,LiBF₃CF₃, LiBF₃C₂F₅, LiBF₃C₃F₇, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂,LiBF₂(CF₃SO₂)₂, and LiBF₂(C₂F₅SO₂)₂. Among them, LiBF₄ is morepreferable from the viewpoint of an improving effect on the initialcharging and discharging efficiency and high temperature cyclecharacteristics.

The borates may be used singly, or in an optional combination of two ormore kinds thereof at an optional ratio.

The amount of the borate (total amount in the case of 2 or more kinds)may be 0.05% by mass or more, is preferably 0.1% by mass or more, morepreferably 0.2% by mass or more, further preferably 0.3% by mass ormore, and particularly preferably 0.4% by mass or more; and may be 10.0%by mass or less, is preferably 5.0% by mass or less, more preferably3.0% by mass or less, further preferably 2.0% by mass or less, andparticularly preferably 1.0% by mass or less. Within the range, a sidereaction of the battery negative electrode is inhibited, so thatincreased in the resistance is suppressed.

[1-4-11-2. Oxalate]

There is no particular restriction on the oxalate, insofar as it is acompound having at least one oxalic acid structure in the molecule. Itcan improve the durability of a battery of the present invention.

As the oxalate, a metal salt represented by the Formula (9) ispreferable. This salt is a salt having an oxalato complex as an anion.M¹ _(a)[M²(C₂O₄)_(b)R_(c) ⁹¹]d  (9)

In Formula (9), M¹ is an element selected from the set consisting ofGroup 1, and Group 2 in the periodic table, and aluminum (Al), and M² isan element selected from the set consisting of transition metals, Group13, Group 14, and Group 15 of the periodic table. R⁹¹ is a groupselected from the set consisting of a halogen, an alkyl group having acarbon number of 1 to 11, and a halogen-substituted alkyl group having acarbon number of 1 to 11; a and b are a positive integer; c is 0 or apositive integer; and d is an integer from 1 to 3.

M¹ is preferably lithium, sodium, potassium, magnesium, or calcium fromthe viewpoint of battery characteristics when an electrolyte solutioncontaining an oxalate is used for a lithium secondary battery, andparticularly preferably lithium.

As M² boron or phosphorus is particularly preferable from the viewpointof electrochemical stability when used in a lithium secondary battery.

Examples of R⁹¹ include fluorine, chlorine, a methyl group, atrifluoromethyl group, an ethyl group, a pentafluoroethyl group, apropyl group, an isopropyl group, a butyl group, a sec-butyl group, anda tert-butyl group, and fluorine and a fluoromethyl group arepreferable.

Examples of the metal salt represented by Formula (9) include thefollowing:

a lithium (oxalato)borate, such as lithium difluoro(oxalato)borate, andlithium bis(oxalato)borate;

a lithium (oxalato)phosphate, such as lithiumtetrafluoro(oxalato)phosphate, lithium difluorobis(oxalato)phosphate,and lithium tris(oxalato)phosphate.

Among these, lithium bis(oxalato)borate and lithiumdifluorobis(oxalato)phosphate are preferable, and lithiumbis(oxalato)borate is more preferable.

The oxalates may be used singly, or in an optional combination of two ormore kinds thereof at an optional ratio.

The amount of the oxalate (total amount in the case of two or more) maybe 0.001% by mass or more, is preferably 0.01% by mass or more, morepreferably 0.1% by mass or more, and particularly preferably 0.3% bymass or more; and may be 10% by mass or less, is preferably 5% by massor less, more preferably 3% by mass or less, further preferably 2% bymass or less, and particularly preferably 1% by mass or less. Within therange, the output characteristics, load characteristics, low temperaturecharacteristics, cycle characteristics, high temperature storagecharacteristics, and other characteristics of a secondary battery can beeasily controlled.

[1-4-11-3. Fluorosulfonate]

There is no particular restriction on the fluorosulfonate, insofar as itis a salt having at least one fluorosulfonic acid structure in themolecule. The fluorosulfonate is able to improve the durability of abattery of the present invention.

There is no particular restriction on a counter cation for thefluorosulfonate, and examples thereof include lithium, sodium,potassium, rubidium, cesium, magnesium, calcium, barium, and an ammoniumrepresented by NR¹³¹R¹³²R¹³³R¹³⁴ (wherein R¹³¹ to R¹³⁴ independently area hydrogen atom or an organic group having a carbon number of 1 to 12).As the counter cation, lithium, sodium, and potassium are preferable,and among them lithium is preferable.

Examples of the fluorosulfonate include lithium fluorosulfonate, sodiumfluorosulfonate, potassium fluorosulfonate, rubidium fluorosulfonate,and cesium fluorosulfonate; and lithium fluorosulfonate is preferable.An imide salt having a fluorosulfonic acid structure, such as lithiumbis(fluorosulfonyl)imide can also be used as the fluorosulfonate.

The fluorosulfonates may be used singly, or in an optional combinationof two or more kinds thereof at an optional ratio.

The content of the fluorosulfonate (total amount in the case of two ormore) may be 0.05% by mass or more, is preferably 0.1% by mass or more,more preferably 0.2% by mass or more, further preferably 0.3% by mass ormore, and particularly preferably 0.4% by mass or more; and may be 10%by mass or less, is preferably 8% by mass or less, more preferably 5% bymass or less, further preferably 2% by mass or less, and particularlypreferably 1% by mass or less. Within the range, a side reaction in abattery occurs little, and increase in the resistance is suppressed.

[2. Nonaqueous Electrolyte Secondary Battery]

A nonaqueous electrolyte secondary battery of an embodiment of thepresent invention is a nonaqueous electrolyte secondary battery providedwith a positive electrode with a positive electrode active materialcapable of absorbing and releasing a metal ion, and a negative electrodewith a negative electrode active material capable of absorbing andreleasing a metal ion; and includes a nonaqueous electrolyte solution;

[2-1. Nonaqueous Electrolyte Solution]

The aforedescribed nonaqueous electrolyte solution is used as anonaqueous electrolyte solution. It is also possible that the abovenonaqueous electrolyte solution is mixed with another nonaqueouselectrolyte solution without departing from the gist of the presentinvention.

[2-2. Negative Electrode]

A negative electrode active material to be used in the negativeelectrode will be described below. There is no particular restriction onthe negative electrode active material, insofar as it is capable ofelectrochemically absorbing and releasing a metal ion such as a lithiumion. Specific examples thereof include a carbonaceous material, analloy-based material, and a lithium-containing metal composite oxidematerial. These may be used singly, or in an optional combination of twoor more kinds thereof.

<Negative Electrode Active Material>

Examples of the negative electrode active material include acarbonaceous material, an alloy-based material, and a lithium-containingmetal composite oxide material.

Examples of the carbonaceous material include (1) natural graphite, (2)artificial graphite, (3) amorphous carbon, (4) carbon coated graphite,(5) graphite coated graphite, and (6) resin coated graphite.

(1) Examples of natural graphite include scaly graphite, flaky graphite,earthy graphite and/or graphite particles obtained by a treatment, suchas spheronization, or densification, on the aforedescribed graphite asthe source material. Among these, spherical or ellipsoidal graphiteundergone a spheronization treatment is particularly preferable from theviewpoints of packing property and charge and discharge ratecharacteristics of the particles.

(2) Examples of artificial graphite include those prepared bygraphitization of an organic compound, such as coal tar pitch,coal-derived heavy oil, atmospheric residue oil, petroleum heavy oil, anaromatic hydrocarbon, a nitrogen-containing cyclic compound, asulfur-containing cyclic compound, polyphenylene, poly(vinyl chloride),poly(vinyl alcohol), polyacrylonitrile, poly(vinyl butyral), a naturalpolymer, poly(phenylene sulfide), poly(phenylene oxide), a furfurylalcohol resin, a phenol-formaldehyde resin, and an imide resin, at atemperatures in a range of from usually 2500° C. to usually 3200° C.,and if necessary further performing pulverization and/or classification.At this time, a silicon-containing compound, a boron-containingcompound, or the like can be used as a graphitization catalyst. Also,artificial graphite obtained by graphitization of mesocarbon microbeadsseparated in a process of a heat treatment of pitch may be included. Inaddition, artificial graphite of granulated particles composed ofprimary particles is also included. For example, artificial graphiteparticles are prepared by preparing flat particles by blending acarbonaceous material powder capable of graphitization, such asmesocarbon microbeads, and coke, with a binder capable ofgraphitization, such as tar, and pitch, as well as a graphitizationcatalyst, and graphitizing the blend, followed by if necessarypulverization, and aggregating or binding a plurality of flat particlessuch that orientation planes become non-parallel.

(3) Examples of the amorphous carbon include amorphous carbon particlesobtained by using an easily graphitizable carbon precursor such as tar,and pitch, for the raw material, and heat-treating the same at leastonce in a temperature range where graphitization does not occur (rangeof 400 to 2200° C.), and amorphous carbon particles obtained by using anon-graphitizable carbon precursor such as resin for the raw material,and heat-treating the same.

(4) Examples of the carbon coated graphite include a carbon graphitecomposite in which amorphous carbon has coated core graphite that isnatural graphite and/or artificial graphite prepared by blending naturalgraphite and/or artificial graphite with a carbon precursor that is anorganic compound such as tar, pitch, and resin, and heat-treating theblend at least once in a range of 400 to 2300° C. The composite may takea form where the entire surface or a part thereof is coated, or a formwhere a plurality of primary particles are combined using carbonoriginating from the aforedescribed carbon precursor as a binder. Also acarbon graphite composite may be obtained by reacting natural graphiteand/or artificial graphite with a hydrocarbon gas such as benzene,toluene, methane, propane, and aromatic volatiles at a high temperatureto deposit carbon on a graphite surface (CVD).

(5) Examples of the graphite coated graphite include graphite coatedgraphite in which a graphitized product has coated all of part of thesurface of core graphite that is natural graphite and/or artificialgraphite prepared by blending natural graphite and/or artificialgraphite with a carbon precursor that is an easily graphitizable organiccompound such as tar, pitch, and resin, and heat-treating the blend atleast once in a range of about 2400 to 3200° C.

(6) Examples of the resin coated graphite include resin coated graphitein which a resin, or the like has coated core graphite that is naturalgraphite and/or artificial graphite prepared by blending naturalgraphite and/or artificial graphite with a resin or the like, and dryingthe blend at a temperature below 400° C.

The above carbonaceous materials (1) to (6) may be used singly, or in anoptional combination of two or more kinds thereof at an optional ratio.

There is no particular restriction on an alloy-based material to be usedas a negative electrode active material, insofar as it can occlude andrelease lithium, and it may be elementary lithium, an elementary metalcomposing a lithium alloy, or an alloy; or any compound among an oxide,a carbide, a nitride, a silicide, a sulfide, or a phosphide of theforegoing. As an elementary metal composing a lithium alloy, or analloy, a material containing a metal or metalloid element of group 13 orgroup 14 (that is, carbon is excluded) is preferable, and an elementarymetal of aluminum, silicon, or tin, and an alloy or a compoundcontaining these atoms are more preferable. These may be used singly, orin an optional combination of two or more kinds thereof at an optionalratio.

<Physical Properties of Carbonaceous Material>

When a carbonaceous material is used as a negative electrode activematerial, it should preferably have the following physical properties.

(X-Ray Parameters)

The d value (interlayer distance) of the lattice plane (002) of acarbonaceous material determined by X-ray diffraction according to themethod of The Japan Society for the Promotion of Science is usually0.335 nm or more; and is usually 0.360 nm or less, preferably 0.350 nmor less, and more preferably 0.345 nm or less. The crystallite size (Lc)of the carbonaceous material determined by X-ray diffraction accordingto the method of The Japan Society for the Promotion of Science ispreferably 1.0 nm or more, and particularly preferably 1.5 nm or more.

(Volume-Based Average Particle Size)

The volume-based average particle size of a carbonaceous material is anaverage particle size (median diameter) based on volume obtained by alaser diffraction/scattering method, and it is usually 1 μm or more,preferably 3 μm or more, more preferably 5 μm or more, and particularlypreferably 7 μm or more. Meanwhile, it is usually 100 μm or less,preferably 50 μm or less, more preferably 40 μm or less, furtherpreferably 30 μm or less, and particularly preferably 25 μm or less.

When the volume-based average particle size is below the above range,the irreversible capacity may increase to incur sometime a loss of theinitial battery capacity. Meanwhile, when the same exceeds the aboverange, non-uniform coated surface tends to be formed at the time ofpreparation of an electrode by coating, which may be undesirable in thebattery manufacturing process.

(BET Specific Surface Area)

The BET specific surface area of a carbonaceous material is a specificsurface area value measured using the BET method, and is usually 0.1m²·g⁻¹ or more, preferably 0.7 m²·g⁻¹ or more, more preferably 1.0m²·g⁻¹ or more, and particularly preferably 1.5 m²·g⁻¹ or more.Meanwhile, it is usually 100 m² g⁻¹ or less, preferably 25 m² g⁻¹ orless, more preferably 15 m² g⁻¹ or less, and particularly preferably 10m² g⁻¹ or less.

If the BET specific surface area value is below the range, theacceptability of lithium at the time of charging is likely todeteriorate when used as a negative electrode material, and lithiumtends to precipitate on the surface of an electrode to reduce thestability. On the other hand, if it exceeds the range, when used as anegative electrode material, the reactivity with a nonaqueouselectrolyte solution increases, which tends to increase gas generationand make it difficult to obtain a preferable battery.

<Constitution and Production Method of Negative Electrode>

An electrode may be produced by any publicly known method to the extentthat the effect of the present invention is not significantly impaired.For example, to the negative electrode active material, a binder, asolvent, and if necessary, also a thickener, a conductive material, afiller, and other constituents are added to form a slurry, which iscoated on a collector, dried, and then pressed to form an electrode.

When an alloy-based material is used, a method of forming a thin filmlayer containing the negative electrode active material (negativeelectrode active material layer) by a technique, such as vapordeposition, sputtering, or plating, may be also applied.

(Electrode Density)

Although there is no particular restriction on the structure of anelectrode when the negative electrode active material is formed to anelectrode, the density of the negative electrode active material presenton a collector is preferably 1 g cm⁻³ or more, more preferably 1.2 gcm⁻³ or more, and particularly preferably 1.3 g cm⁻³ or more. Meanwhile,it is preferably 2.2 g cm⁻³ or less, more preferably 2.1 g cm³ or less,further preferably 2.0 g cm³ or less, and particularly preferably 1.9 gcm³ or less.

When the density of the negative electrode active material present onthe collector exceeds the above range, the negative electrode activematerial particles may be destructed to cause increase in the initialirreversible capacity, or deterioration of the charge and dischargecharacteristics at a high current density due to decrease inpermeability of a nonaqueous electrolyte solution into the vicinity ofthe interface between the collector and the negative electrode activematerial. Meanwhile, when the density falls below the range, theconductivity within the negative electrode active material is decreasedto increase the battery resistance, which may reduce the capacity perunit volume.

[2-3. Positive Electrode]

<Positive Electrode Active Material>

In an embodiment of the present invention, a positive electrode activematerial used for a positive electrode contains a lithium transitionmetal compound, and the positive electrode active material contains atleast Ni, Mn, and Co, wherein a molar ratio of Mn/(Ni+Mn+Co) is largerthan 0 and not larger than 0.32, and a molar ratio of Ni/(Ni+Mn+Co) is0.45 or more. The lithium transition metal compound will be describedbelow.

<Lithium Transition Metal Compound>

The lithium transition metal compound is a compound having a structurecapable of desorbing and absorbing Li ions, and for example the lithiumtransition metal compound represented by the following Formula (I) maybe used in the present invention. Further, that belonging to a layeredstructure enabling lithium ions to diffuse two-dimensionally ispreferable. Here, the layered structure will be described in moredetail. As for a typical crystal system having a layered structure,there are LiCoO₂ and LiNiO₂, belonging to the α-NaFeO₂ type, which is ahexagonal system belonging to the following space group in terms ofsymmetry:R3 m(hereinafter sometimes denoted as “layered R(−3)m structure”).

However, the layered LiMeO₂ is not limited to the layered R(−3)mstructure. Besides this, LiMnO₂ which is so-called layered Mn is alayered compound belonging to an orthorhombic system with the spacegroup Pm2m, and Li₂MnO₃ which is so-called 213 phase may be also denotedas Li[Li_(1/3)Mn_(2/3)]O₂, and is belonging to a monoclinic system withthe space group C₂/m structure, is also a layered compound in which a Lilayer, a Li[Li_(1/3)Mn_(2/3)] layer, and an oxygen layer are stacked.

The lithium transition metal compound preferably contains a lithiumtransition metal compound represented by the following compositionalformula (I), and more preferably is a lithium transition metal compoundrepresented by the following compositional formula (I).Li_(1+x)MO₂  (I)

In Formula (I), x is usually from −0.20 to 0.50. Particularly, the lowerlimit value of x is preferably −0.05 or more, more preferably −0.03 ormore, especially preferably −0.02 or more, and most preferably −0.01 ormore. Meanwhile, the upper limit of x may be 0.1 or less, is preferably0.06 or less, more preferably 0.028 or less, further preferably 0.020 orless, especially preferably 0.010 or less, and most preferably 0.005 orless. It is preferable that x is within the above range, because thesuppression effect on gas generation can be sufficiently exhibited todevelop an adequate charge/discharge capacity by combination of amonofluorophosphate and/or a difluorophosphate contained in anelectrolyte solution.

Furthermore, in Formula (I), M is composed of at least Ni, Mn, and Co,and the molar ratio of Mn/(Ni+Mn+Co) is larger than 0 and not largerthan 0.32.

The lower limit of the molar ratio of Mn/(Ni+Mn+Co) is preferably 0.05or more, more preferably 0.08 or more, further preferably 0.10 or more,particularly preferably 0.12 or more, and most preferably 0.14 or more.The upper limit of the molar ratio of Mn/(Ni+Mn+Co) is preferably 0.28or less, more preferably 0.26 or less, further preferably 0.25 or less,particularly preferably 0.24 or less, and most preferably 0.23 or less.

When the molar ratio of Mn/(Ni+Mn+Co) is within the above range, theportion of Mn that does not participate in charge and discharge issufficiently small, and the battery can have a high capacity, which ispreferable.

Furthermore, the lower limit of the molar ratio of Ni/(Ni+Mn+Co) is 0.45or more, preferably 0.50 or more, and more preferably 0.55 or more. Theupper limit of the molar ratio of Ni/(Ni+Mn+Co) is usually 0.95 or less,may be 0.85 or less, is preferably 0.80 or less, more preferably 0.75 orless, further preferably 0.70 or less, particularly preferably 0.68 orless, and most preferably 0.64 or less.

When the molar ratio of Ni/(Ni+Mn+Co) is within the above range, theportion of Ni involved in charge and discharge is sufficiently high, andthe battery can have a high capacity, which is preferable.

Furthermore, although there is no particular restriction on the lowerlimit of the molar ratio of Co/(Ni+Mn+Co), it is preferably 0.05 ormore, more preferably 0.08 or more, further preferably 0.10 or more, andparticularly preferably 0.15 or more. The upper limit of the molar ratioof Co/(Ni+Mn+Co) is also not particularly limited, but it is preferably0.33 or less, more preferably 0.30 or less, further preferably 0.28 orless, particularly preferably 0.26 or less, and most preferably 0.24 orless.

It is preferable that the molar ratio of Co/(Ni+Mn+Co) is in the aboverange because the charge/discharge capacity becomes large.

In the compositional formula (I), although the atomic ratio of theoxygen amount is described as 2 for convenience, there may be somenon-stoichiometry. Meanwhile, x in the above Formula (I) is thecomposition of a supply in the manufacture stage of a lithium transitionmetal compound. Usually, batteries on the market have been aged afterthey are assembled to batteries. Therefore, due to charge and discharge,the Li amount in the positive electrode may be deficient.

Also, a hetero element may be introduced into a lithium transition metalcompound. The hetero element is one or more kinds selected from B, Na,Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Sr, Y, Zr, Nb, Ru, Rh, Pd, Ag, In,Sb, Te, Ba, Ta, Mo, W, Re, Os, Ir, Pt, Au, Pb, La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Bi, N, F, S, Cl, Br, I, As, Ge, P, Pb,Sb, Si, and Sn. Among them, at least one element selected from the groupconsisting of Fe, Cu, W, Mo, Nb, V, Ta, Mg, Al, Ti, Zr, Zn, Ca, Be, B,Bi, Li, Na, and K is preferable.

The hetero element may be introduced into the crystal structure of alithium transition metal compound, or not introduced into the crystalstructure of a lithium transition metal compound, but rather localizedon the particle surface or crystal grain boundary as a simple substanceor a compound.

(Physical Properties of Positive Electrode Active Material ContainingLithium Transition Metal Compound, or Lithium Transition Metal Compound)

(1) Sulfate Salt

A positive electrode active material may contain a sulfate salt.Although there is no particular restriction on the content of a sulfatesalt to be contained in a positive electrode active material, it ispreferably 15 μmol/g or more from the viewpoint of exhibition of thesuppression effect on gas generation by a monofluorophosphate and/or adifluorophosphate.

Further, it is more preferably 20 μmol/g or more, further preferably 25μmol/g or more, particularly preferably 32 μmol/g or more, and mostpreferably 35 μmol/g or more. Further, the upper limit is preferably 100μmol/g or less, more preferably 80 μmol/g or less, further preferably 60μmol/g or less, particularly preferably 50 μmol/g or less, and mostpreferably 30 μmol/g or less, because gas generation is increased due toa side reaction.

The sulfate salt content in a positive electrode active material can bemeasured, for example, by water extraction ion chromatography.

(2) Average Valence of Ni

Although there is no particular restriction on the average valence of Niof the lithium transition metal compound contained in a positiveelectrode active material in an uncharged state, it is preferably 2.1 ormore because the Ni portion can be increased so that the battery canhave high capacity. Further, it is more preferably 2.3 or more, furtherpreferably 2.5 or more, particularly preferably 2.55 or more, and mostpreferably 2.6 or more. Meanwhile, the upper limit is preferably 3 orless, more preferably 2.9 or less, particularly preferably 2.8 or less,and most preferably 2.7 or less, otherwise the structural stability ofthe active material is lowered.

Here, the Ni valence in the present invention will be described indetail.

First, when the compositional formula of the lithium transition metalcompound is rewritten to the following compositional formula (I′), M′ iscomposed of Li, Ni, and Mn, or Li, Ni, Mn, and Co.LiM′O₂  (I′)

Although the atomic ratio with respect to the oxygen amount is describedin the above Formula (I′) as 2 for convenience, but there may be somenon-stoichiometry. When there is non-stoichiometry, the atomic ratio ofoxygen is usually in a range of 2±0.2, preferably in a range of 2±0.15,more preferably in a range of 2±0.12, further preferably in a range of2±0.10, and particularly preferably in a range of 2±0.05.

Furthermore, it is particularly preferable that a lithium transitionmetal compound has an atomic composition represented by the followingFormula (II) in the site of M′ in Formula (I′).M′=Li_(z/(2+z))[(Ni_((1+y)/2)Mn_((1−y/2)))_(1−x)CO_(x)]_(2/(2+z))  (II)

Here, the chemical meaning of the Li composition (z and x) in a lithiumnickel manganese cobalt composite oxide, which is a preferredcomposition of a lithium transition metal compound, will be described inmore detail below.

As described above, the layered structure is not necessarily limited tothe R(−3)m structure, but it is preferable from the viewpoint ofelectrochemical performance that the layered structure is attributableto the R(−3)m structure.

In order to find x, y, and z in the compositional formula of a lithiumtransition metal compound, x, y, and z are obtained by a calculationbased on the Li/Ni/Mn/Co ratio of the lithium transition metal compoundto be determined by analyzing each transition metal and Li by aninductively coupled plasma atomic emission spectrophotometer (ICP-AES),and by analyzing Li in the surface impurities by water extraction ionchromatography.

From a structural viewpoint, it is considered that Li related to z isintroduced in the same transition metal site by substitution. In thiscase, by Li related to z, the average valence of Ni becomes larger than2 (trivalent Ni is formed) by the principle of charge neutrality.

Since z increases the Ni average valence, it can be an index of Nivalence (the proportion of Ni (III)).

From the above compositional formula, when calculating a Ni valence (m)along with changes in z and z′ assuming that the Co valence is 3 and theMn valence is 4, it comes to:

$m = {2\left\lbrack {2 - \frac{1 - x - z}{\left( {1 - x} \right)\left( {1 + y} \right)}} \right\rbrack}$This calculation result means that the Ni valence is not determined onlyby z, but is a function of x and y. If z=0 and y=0, the Ni valenceremains 2 regardless of the value of x. If z is negative, it means thatthe amount of Li contained in the active material is less than thestoichiometric amount, and in the case of a very large negative value,there is a possibility that the effect of the present invention cannotbe obtained. Meanwhile, even if the z value is the same, the Ni valencebecomes high in the case of a Ni-rich (large y value), and/or a Co-rich(large×value) composition, and when used in a battery, the ratecharacteristics and output characteristics increase, but on the otherhand, the capacity tends to decrease. From this, it can be said that itis more preferable to define the upper limit and the lower limit of thez value as a function of x and y.

Further, when the x value is 0≤x≤0.1, and the amount of Co is in a smallrange, the cost is reduced, and in addition the charge/dischargecapacity, cycle characteristics, and safety are improved, when used in alithium secondary battery which is designed such that charging iscarried out at a high charging potential.

(3) pH

Although there is no particular restriction on the pH of an aqueoussolution of a lithium transition metal compound, it is preferably 11 ormore on the basis of the liquid temperature of 25° C., because theeffect of suppressing gas generation by the combination of amonofluorophosphate and/or a difluorophosphate contained in anelectrolyte solution tends to be sufficiently exhibited. Further, the pHis more preferably 11.2 or more on the basis of the liquid temperatureof 25° C., more preferably 11.4 or more, particularly preferably 11.6 ormore, and most preferably 11.8 or more. Further, the upper limit ispreferably 13 or less on the basis of the liquid temperature of 25° C.,more preferably 12.7 or less, particularly preferably 12.4 or less, andmost preferably 12 or less, because gas generation due to a sidereaction is reduced.

As a method for measuring the pH of the above lithium transition metalcompound, 50 g of demineralized water is weighed into a beaker, and 5 gof a sample is added with stirring, while monitoring the liquidtemperature and pH value, and the values of the pH and liquidtemperature measured after 10 min from the addition are used.

(4) Carbonate Salt

A positive electrode active material may contain a carbonate salt.Although there is no particular restriction on the content of acarbonate salt that can be contained in a positive electrode activematerial, it is preferably 10 μmol/g or more, because the effect ofsuppressing gas generation by the combination of a monofluorophosphateand/or a difluorophosphate contained in an electrolyte solution tends tobe sufficiently exhibited. The content is more preferably 20 μmol/g ormore, further preferably 40 μmol/g or more, particularly preferably 60μmol/g or more, and most preferably 80 μmol/g or more. Further, theupper limit is preferably 100 μmol/g or less, more preferably 98 μmol/gor less, particularly preferably 96 μmol/g or less, and most preferably94 μmol/g or less, because gas generation due to a side reaction isreduced.

The amount of a carbonate salt contained in the lithium transition metalcompound can be measured, for example, by water extraction ionchromatography.

(5) Tap Density

A lithium transition metal compound composing a positive electrodeactive material is usually a powder, and its tap density is notparticularly restricted, however it is preferably 1.8 g/cm³ or morebecause, when assemble to a battery, the charge/discharge capacitybecomes high. It is more preferably 2 g/cm³ or more, further preferably2.1 g/cm³ or more, particularly preferably 2.2 g/cm³ or more, and mostpreferably 2.3 g/cm³ or more. Meanwhile, the upper limit is preferably4.0 g/cm³ or less, more preferably 3.8 g/cm³ or less, particularlypreferably 3.6 g/cm³ or less, and most preferably 3.4 g/cm³ or less,because the output characteristics can be sufficient.

When a lithium transition metal compound having a high tap density isused, a positive electrode with a high density can be formed. When thetap density of a lithium transition metal compound is within the aboverange, the amount of a dispersion medium necessary in forming a positiveelectrode becomes reasonable, and therefore the amounts of a conductivematerial and a binder become also reasonable, so that the filling rateof the positive electrode with a lithium transition metal compound isnot restricted, and the influence on the battery capacity is alsoreduced.

For measuring of the tap density of a lithium transition metal compound,a sample is made to pass a sieve with a mesh opening of 300 μm and todrop into a 20 cm³ tapping cell to fill its volume, and tapped 200 timeswith a stroke length of 10 mm using a powder density analyzer (forexample, Tap Denser manufactured by Seishin Enterprise Co., Ltd.), andthe density may be calculated from the then volume and the sample mass.

Alternatively, for simplicity, the sample is dropped in a 10 mLgraduated cylinder to fill its volume, followed by tapping 200 times,and the density may be calculated from the then volume and the samplemass.

(6) Surface Coating

The above lithium transition metal compound, on which surface asubstance having a composition different from that of the substancemainly composing the lithium transition metal compound (hereinafterreferred to at pleasure as “surface adhering substance”) is adhered, mayalso be used. Examples of the surface adhering substance include oxides,such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfate salts, such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate;carbonate salts, such as lithium carbonate, calcium carbonate, andmagnesium carbonate; and carbon.

These surface adhering substances can be made to adhere to the surfaceof a lithium transition metal compound, for example, by a method inwhich they are dissolved or suspended in a solvent, and a lithiumtransition metal compound is impregnated therewith and then dried; amethod in which a precursor of a surface adhering substance is dissolvedor suspended in a solvent, and a lithium transition metal compound isimpregnated therewith followed by reaction by heating or other means; ora method in which the same is added to a precursor of a lithiumtransition metal compound, and both are fired at the same time. Whencarbon is adhered, a method in which a carbonaceous substance is stuckmechanically, for example, in the form of activated carbon afterward,may be also applied.

The mass of a surface adhering substance adhered to the surface of alithium transition metal compound is preferably 0.1 ppm or more withrespect to the mass of the lithium transition metal compound, morepreferably 1 ppm or more, and further preferably 10 ppm or more.Meanwhile, it is preferably 20% or less, more preferably 10% or less,and further preferably 5% or less.

An oxidation reaction of a nonaqueous electrolyte solution on thesurface of a lithium transition metal compound can be suppressed by thesurface adhering substance, so that the battery life can be prolonged.Further, when the adhered amount is within the above range, the effectcan be sufficiently exhibited, and incoming and outgoing of lithium ionsare not impeded, and the resistance is suppressed from increasing.

(7) Shape

As for the shape of a lithium transition metal compound, a conventionalshape, such as massive, polyhedral, spherical, ellipsoidal, tabular,acicular, or columnar, may be used. Further, primary particles may beaggregated to form secondary particles, and the shape of which may bespherical or ellipsoidal.

(8) Median Diameter d50

The median diameter d50 (of the secondary particle size, when primaryparticles are aggregated to form secondary particles) of a lithiumtransition metal compound can be measured using a laserdiffraction/scattering particle size distribution analyzer.

The median diameter d50 is preferably 0.1 μm or more, more preferably0.5 μm or more, further preferably 1 μm or more, and particularlypreferably 3 μm or more, while it is preferably 30 μm or less, morepreferably 20 μm or less, further preferably 16 μm or less, andparticularly preferably 15 μm or less. When the median diameter d50 iswithin the above range, it is easy to obtain a high bulk densityproduct, and further it does not take time for lithium to diffuse in aparticle, so that the battery characteristics are not easilydeteriorated. In addition, at the time of production of the positiveelectrode of a battery, that is, when an active material together with aconductive material, a binder, or the like is slurried using a solventand applied into a thin film, streaking are less likely to occur.

In this regard, by mixing two or more kinds of lithium transition metalcompounds having different median diameters d50 at an optional ratio,the packing property at the time of preparing a positive electrode canalso be improved.

A measurement of the median diameter d50 of a lithium transition metalcompounds is carried out using a 0.1% by mass sodium hexametaphosphateaqueous solution as a dispersion medium with a particle sizedistribution meter (for example LA-920, manufactured by Horiba, Ltd.Co., Ltd.) after ultrasonic dispersion of the dispersion of lithiumtransition metal compound for 5 min and setting a measurement refractiveindex at 1.24.

(9) Average Primary Particle Size

When primary particles are aggregated to form secondary particles, theaverage primary particle size of a lithium transition metal compound ispreferably 0.01 μm or more, more preferably 0.05 μm or more, furtherpreferably 0.08 μm or more, and particularly preferably 0.1 μm or more,meanwhile is preferably 3 μm or less, more preferably 2 μm or less,further preferably 1 μm or less, and particularly preferably 0.6 μm orless. Within the above range, it is easy to form spherical secondaryparticles, so that the powder packing property becomes appropriate,while sufficiently securing a specific surface area, and thereforedeterioration of the battery performance such as output characteristicscan be suppressed.

The average primary particle size of a lithium transition metal compoundis measured by observation using a scanning electron microscope (SEM).Specifically, it can be obtained by measuring the largest segment lengthof a lateral line defined by opposing boundary lines of a primaryparticle with respect to optional 50 primary particles in a photographat a magnification of 10000×, and taking the average value.

(10) BET Specific Surface Area

As for the BET specific surface area of a lithium transition metalcompound, the specific surface area measured by a BET method ispreferably 0.2 m²·g⁻¹ or more, more preferably 0.3 m²·g⁻¹ or more, andfurther preferably 0.4 m²·g⁻¹ or more, meanwhile, it is preferably 4.0m²·g⁻¹ or less, more preferably 2.5 m²·g⁻¹ or less, and furtherpreferably 1.5 m²·g⁻¹ or less. When the value of the BET specificsurface area is within the above range, it is easy to prevent decreasein battery performance. Furthermore, sufficient tap density can besecured, so that the coating property at the time of forming a positiveelectrode is improved.

The BET specific surface area of a lithium transition metal compound ismeasured using a surface area meter (for example, the fully automaticsurface area measuring instrument manufactured by Ohkura Riken Co.Ltd.). Specifically, after preliminary drying in which a sample is driedin a nitrogen stream at 150° C. for 30 min, the specific surface area ismeasured by a nitrogen adsorption BET one-point method according to aflowing gas method using a nitrogen and helium mixed gas accuratelyregulated at a nitrogen relative pressure of 0.3 with respect to theatmospheric pressure. The specific surface area obtained by thismeasurement is defined as the BET specific surface area of a lithiumtransition metal compound in the present invention.

(Method for Producing Positive Electrode Active Material ContainingLithium Transition Metal Compound)

There is no particular restriction on the method for producing apositive electrode active material containing a lithium transition metalcompound, insofar as it does not exceed the gist of the presentinvention, and a common method as a production method for an inorganiccompound may be used among many methods.

Although various methods are conceivable for preparing a positiveelectrode active material having a spherical or ellipsoidal form, thereis for example a method, in which a transition metal source material,such as a nitrate salt, or a sulfate salt of a transition metal, and ifnecessary a source material with another element are dissolved, ordisintegrated and dispersed in a solvent such as water, and the pHthereof is adjusted with stirring to yield and collect a sphericalprecursor. This product is dried as necessary, and fired at a hightemperature after adding a Li source, such as LiGH, Li₂CO₃, and LiNO₃ toyield a positive electrode active material.

Examples of other method include a method in which a transition metalsource material such as a nitrate salt, a sulfate salt, a hydroxide, andan oxide of a transition metal and if necessary a source material withanother element are dissolved, or disintegrated and dispersed in asolvent such as water, and dried and shaped by a spray dryer, or thelike into a spherical or ellipsoidal precursor, and a Li source, such asLiGH, Li₂CO₃, and LiNO₃ is added thereto and the mixture is fired at ahigh temperature to yield a positive electrode active material.

Examples of other method include a method in which a transition metalsource material such as a nitrate salt, a sulfate salt, a hydroxide, andan oxide of a transition metal, a Li source, such as LiGH, Li₂CO₃, andLiNO₃, and if necessary a source material with another element aredissolved, or disintegrated and dispersed in a solvent such as water,and dried and shaped by a spray dryer, or the like into a spherical orellipsoidal precursor, which is then fired at a high temperature toyield a positive electrode active material.

Meanwhile, the sulfate salt content or the carbonate salt content in theaforedescribed positive electrode active material may be regulated to adesired value by adjusting the amounts of a sulfate salt and a carbonatesalt used in selecting a transition metal source material, or the firingtemperature, or by performing washing or not.

A lithium transition metal compound used for positive electrode activematerial may be used singly or in a blend of two or more kinds. Further,it may be blended with a sulfide, or a phosphate compound, anotherlithium transition metal composite oxide, or the like. Examples of asulfide include compounds having a two-dimensional layered structure,such as TiS₂ and MoS₂, and compounds represented by the general formulaMe_(x)Mo₆S₈ (Me is various transition metals, such as Pb, Ag, and Cu).Examples of a phosphate compound include those belonging to the olivinestructure, and are generally represented by LiMePO₄ (Me is at least onetransition metal), and specific examples thereof include LiFePO₄,LiCoPO₄, LiNiPO₄, and LiMnPO₄. Examples of the lithium transition metalcomposite oxide include those belonging to a spinel structure allowingthree-dimensional diffusion, and a layered structure allowingtwo-dimensional diffusion of lithium ions. Those having a spinelstructure are generally expressed as LiMe₂O₄ (Me is at least onetransition metal), and specific examples thereof include LiMn₂O₄,LiCoMnO₄, LiNi_(0.5)Mn_(1.5)O₄, and LiCoVO₄. Specific examples of thosehaving a layered structure include LiCoO₂, LiNiO₂, LiNi_(1-x)Co_(x)O₂,LiNi_(1-x-y)Co_(x)Mn_(y)O₂, LiNi_(0.5)Mn_(0.5)O₂,Li_(1.2)Cr_(0.4)Mn_(0.4)O₂, Li_(1.2)Cr_(0.4)Ti_(0.4)O₂, and LiMnO₂.

<Constitution and Production Method of Positive Electrode for LithiumSecondary Battery>

The positive electrode for a lithium secondary battery is so constructedthat a layer of a positive electrode active material containing a binderand a positive electrode active material containing the aforedescribedlithium transition metal compound is formed on a collector.

A layer of a positive electrode active material is produced by blendinga positive electrode active material containing a lithium transitionmetal compound, with a binder, if necessary together with a conductivematerial, a thickener, or the like, in a dry state and formed into asheet form, and press-bonding it to a positive electrode collector; orby dissolving or dispersing these source materials in a liquid medium toform a slurry, and coating it to a positive electrode collector, andfollowed by drying.

As the material for a positive electrode collector, usually a metalmaterial, such as aluminum, stainless steel, nickel plating, titanium,and tantalum, or a carbon material, such as carbon cloth and carbonpaper is used. Examples of the form thereof include, in the case of themetal material, a metal foil, a metal cylinder, a metal coil, a metalsheet, a metal thin film, an expanded metal, a punched metal, and ametal foam; and in the case of the carbon material, a carbon sheet, acarbon thin film, and a carbon cylinder.

In this regard, a thin film may be appropriately formed into a meshform.

Although there is no particular restriction on a binder to be used inthe production of a layer of a positive electrode active materialinsofar as it is a stable material with respect to a liquid medium to beused at the time of production of an electrode in the case of a coatingmethod, specific examples thereof include a resin type polymer, such aspolyethylene, polypropylene, poly(ethylene terephthalate), poly(methylmethacrylate), an aromatic polyamide, cellulose, and nitrocellulose; arubber type polymer, such as SBR (styrene/butadiene rubber), NBR(acrylonitrile/butadiene rubber), fluorocarbon rubber, isoprene rubber,butadiene rubber, and ethylene/propylene rubber; a thermoplasticelastomer polymer, such as a styrene/butadiene/styrene block copolymerand its hydrogenated product, EPDM (ethylene/propylene/diene terpolymerpolymer), a styrene/ethylene/butadiene/ethylene copolymer, and astyrene/isoprene/styrene block copolymer and its hydrogenated product; asoft resin type polymer, such as syndiotactic 1,2-polybutadiene,poly(vinyl acetate), an ethylene/vinyl acetate copolymer, and apropylene/α-olefin copolymer; a fluorocarbon resin, such aspoly(vinylidene fluoride), polytetrafluoroethylene, fluorinatedpoly(vinylidene fluoride), a tetrafluoroethylene/ethylene copolymer; anda polymer composition having the ion conductivity of alkali metal ions(especially lithium ions).

These substances may be used singly, or in an optional combination oftwo or more kinds thereof at an optional ratio.

The content of a binder in a positive electrode active material layer isusually 0.1% by mass or more, and 80% by mass or less. When the contentof the binder is too low, a lithium transition metal compound cannot besufficiently retained, and the mechanical strength of a positiveelectrode becomes insufficient, and the battery performance such ascycle characteristics may be deteriorated. On the other hand, when it istoo high, the battery capacity and conductivity may be reduced.

A conductive material is usually added to a positive electrode activematerial layer in order to enhance the conductivity.

Although there is no particular restriction on its type, specificexamples thereof include a metal material, such as copper and nickel;and a carbonaceous material, such as graphite including naturalgraphite, and artificial graphite, carbon black including acetyleneblack, and amorphous carbon including needle coke.

These substances may be used singly, or in an optional combination oftwo or more kinds thereof at an optional ratio.

The content of the conductive material in a positive electrode activematerial is usually 0.01% by mass or more, and 50% by mass or less. Whenthe content of the conductive material is too low, the conductivity maybecome insufficient; conversely, when it is too high, the batterycapacity may sometimes decrease.

There is no particular restriction on the type of a liquid medium forforming a slurry, insofar as it is a solvent capable of dissolving ordispersing a positive electrode active material containing a lithiumtransition metal compound which is a positive electrode material, abinder, and a conductive material and a thickener to be used asnecessary, and either of an aqueous solvent and an organic solvent maybe used. Examples of the aqueous solvent include water and alcohol.Examples of the organic solvent include N-methylpyrrolidone (NMP),dimethylformamide, dimethylacetamide, methyl ethyl ketone,cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine,N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF),toluene, acetone, dimethyl ether, dimethylacetamide,hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline,pyridine, methylnaphthalene, and hexane. In particular, when the aqueoussolvent is used, it is slurried by adding a dispersing agent togetherwith a thickener, and using a latex such as SBR.

Meanwhile, these solvents may be used singly, or in an optionalcombination of two or more kinds thereof at an optional ratio.

The content of a lithium transition metal compound as a positiveelectrode material in a positive electrode active material layer isusually 10% by mass or more, and 99.9% by mass or less. When the portionof a lithium transition metal compound in a positive electrode activematerial layer is too high, the strength of a positive electrode tendsto be insufficient, and when the proportion is too low, the capacity maysometimes become insufficient.

The thickness of a positive electrode active material layer is usuallyabout 10 to 200 μm.

Here, the plate density of a positive electrode according to the presentinvention is 3.0 g/cm³ or more. Further, it is preferably 3.2 g/cm³ ormore, more preferably 3.3 g/cm³ or more, further preferably 3.4 g/cm³ ormore, and particularly preferably 3.6 g/cm³ or more. The upper limit ispreferably 4.2 g/cm³ or less because deterioration of the input/outputcharacteristics does not occur easily, more preferably 4.1 g/cm³ orless, particularly preferably 4.0 g/cm³ or less, and most preferably 3.9g/m³ or less.

Increase of the plate density of a positive electrode up to the aboverange can be attained through compaction by roll pressing a positiveelectrode active material layer after coating and drying. A desiredplate density can be obtained by appropriately regulating the pressureof the roll press.

[2-4. Separator]

A separator is usually interposed between a positive electrode and anegative electrode to prevent a short circuit. In this case, thenonaqueous electrolyte solution is usually used to impregnate theseparator.

There is no particular restriction on the material and the shape of aseparator, and any publicly known ones may be arbitrarily adopted to theextent that the effect of the present invention is not significantlyimpaired. Among others, it is preferable to use a separator in a form ofa porous sheet or a nonwoven fabric which is superior in liquidretention and made of a material stable with respect to the nonaqueouselectrolyte solution, for example, resin, glass fiber, inorganicmaterial.

As the material for a separator made of a resin or a glass fiber, forexample, polyolefin such as polyethylene or polypropylene, an aromaticpolyamide, polytetrafluoroethylene, polyethersulfone, sintered glass orthe like may be used. Among these, sintered glass and polyolefin arepreferable, and polyolefin is more preferable. These materials may beused singly, or in an optional combination of two or more kinds thereofat an optional ratio.

Although the thickness of a separator is arbitrary, it is usually 1 μmor more, preferably 5 μm or more, and more preferably 10 μm or more.Meanwhile, it is usually 50 μm or less, preferably 40 μm or less, andmore preferably 30 μm or less.

When the separator is thinner than the above range, the insulation andmechanical strength may sometimes decrease. Also, when it is thickerthan the above range, not only the battery performance such as ratecharacteristics may sometimes be deteriorated, but also the energydensity of a nonaqueous electrolyte secondary battery as a whole maysometimes decrease.

Furthermore, when a porous material such as a porous sheet or nonwovenfabric is used as a separator, the porosity of the separator isarbitrary, but is usually 20% or more, preferably 35% or more, and morepreferably 45% or more. Meanwhile, it is usually 90% or less, preferably85% or less, and more preferably 75% or less.

When the porosity is smaller than the above range, the film resistancetends to increase, and the rate characteristics tend to deteriorate.Meanwhile, when it is larger than the above range, the mechanicalstrength of a separator decreases and the insulation property tends todecrease.

The average pore diameter of a separator is also arbitrary, but isusually 0.5 μm or less, and preferably 0.2 μm or less. Meanwhile, it isusually 0.05 μm or more. When the average pore diameter exceeds theabove range, a short circuit tends to occur. When the value is below theabove range, the film resistance increases and the rate characteristicsmay sometimes decrease.

Meanwhile, as an inorganic material, for example, an oxide, such asalumina and silicon dioxide, a nitride, such as aluminum nitride, andsilicon nitride, and a sulfate salt, such as barium sulfate and calciumsulfate, are used, and particularly those in a particulate form, or afibrous form are used.

As for the form of a separator, that having a thin film form, such as anonwoven fabric, a woven fabric, or a microporous film, is used. As thethin film form, that having a pore size of 0.01 to 1 μm and a thicknessof 5 to 50 μm is favorably used. In addition to the aforedescribedindependent thin film form, a separator prepared by forming compositeporous layers containing inorganic particles described above on theouter surfaces of the positive electrode and/or the negative electrodeusing a resin binder may be used. For example, a porous layer may beformed on each side of the positive electrode with alumina particleshaving a 90% particle size of less than 1 μm using a fluorocarbon resinas a binder.

A characteristic of a separator in a nonelectrolyte secondary batterymay be grasped by the Gurley value. The Gurley value indicates thedifficulty of air permeability in the thickness direction of a film, andexpressed by the number of seconds required for 100 mL of air to passthrough the film. Therefore, a smaller value means easier permeation,and a higher value means more difficult permeation. In other words, asmaller value means that the communication in the film thicknessdirection is better, and a larger value means that the communication inthe film thickness direction is worse. The communication means thedegree of interconnection between pores in the film thickness direction.When the Gurley value of a separator of the present invention is low, itmay be used in various applications. For example, in a case where theseparator is used as a separator for a non-aqueous lithium secondarybattery, when the Gurley value is low, the movement of lithium ions iseasy, and the battery performance can be excellent, which is preferable.Although the Gurley value of the separator may be arbitrary, it ispreferably from 10 to 1000 sec/100 mL, more preferably from 15 to 800sec/100 mL, and further preferably from 20 to 500 sec/100 mL. When theGurley value is 1000 sec/100 mL or less, the electrical resistance issubstantially low, which is preferable for a separator.

[2-5. Battery Design]

<Electrode Group>

The electrode group may be either of that with a layered structure ofthe aforedescribed positive electrode plate and negative electrode plateinterposing the aforedescribed separator, and that with a structurewinding spirally the aforedescribed positive electrode plate andnegative electrode plate interposing the aforedescribed separator.

The ratio of the volume of the electrode group to the internal volume ofthe battery (hereinafter referred to as “electrode group occupancy”) isusually 40% or more, and preferably 50% or more. Meanwhile, it isusually 90% or less, and preferably 80% or less.

When the electrode group occupancy falls below the above range, thebattery capacity decreases. While, when it exceeds the above range, thevacant space becomes small, and if the battery temperature increases,the components expand, or the vapor pressure of the liquid component ofthe electrolyte increases to raise the internal pressure, which maydeteriorate various battery characteristics such as charge and dischargerepeatability and high temperature storage property, or activate a gasrelease valve to release the internal pressure outward.

<Protection Device>

As a protection device, a PTC (Positive Temperature Coefficient)element, which resistance increases when abnormal heat generationoccurs, or overcurrent flows, a thermal fuse, a thermistor, a valve(current cutoff valve) that cuts off the current flowing in a circuitactivated by sudden rise of the internal pressure or the internaltemperature of the battery at the occasion of abnormal heat generation,or the like may be used. It is preferable to select a protection devicefrom the above that is not activated under the condition of normal useof high current, and a design which does not allow occurrence ofabnormal heat generation or thermal runaway even without a protectiondevice is more preferable.

<Outer Package>

A nonaqueous electrolyte secondary battery of the present invention isusually so constructed that the nonaqueous electrolyte solution, thenegative electrode, the positive electrode, the separator, and othercomponents are contained in an outer package (outer case). There is noparticular restriction on the outer package, and any publicly known onecan be arbitrarily adopted insofar as the effect of the presentinvention is not significantly impaired. Specifically, although thematerial of the outer package is arbitrary, usually, for example, anickel-plated steel sheet, stainless steel, aluminum or an alloythereof, a magnesium alloy, a metal such as nickel or titanium, or alaminated film of a resin and an aluminum foil (laminate film) isfavorably used.

Examples of an outer case using the metal include that having ahermetically sealed structure formed by welding the metal members bylaser welding, resistance welding, or ultrasonic welding, and thathaving a crimped structure with the metal members interposing a resingasket.

Examples of an outer case using the laminate film include that having ahermetically sealed structure formed by heat-sealing the resin members.In order to improve the heat sealability, a resin different from theresin used for the laminate film may be interposed between the resinlayers.

Especially, when the resin layers are heat-sealed interposing acollector terminal to form a hermetically sealed structure, since ametal to resin bonding is to be formed, a resin having polar groups, ora modified resin that has introduced polar groups may be favorably usedas the interposed resin.

The shape of the outer package is also arbitrary, and may be any of acylindrical shape, a square shape, a laminate form, a coin shape, largesize, for example.

<Battery Voltage>

A nonaqueous electrolyte secondary battery of the present invention isusually used at a battery voltage of 4.0 V or higher. The batteryvoltage is preferably 4.1 V or higher, more preferably 4.15 V or higher,and most preferably 4.2 V or higher. This is because by increasing thebattery voltage, the energy density of the battery can be increased.

On the other hand, when the battery voltage is raised, the potential ofthe positive electrode rises, and there arises a problem that a sidereaction on the surface of the positive electrode increases. When abattery of the present invention is used, the above problem can besolved, however, if the voltage is too high, the side reaction amount onthe surface of the positive electrode will become too large, and thebattery characteristics will be deteriorated. Therefore, the upper limitof the battery voltage is preferably 5 V or less, more preferably 4.8 Vor less, and most preferably 4.6 V or less.

<Reason Behind Exhibition of Effect of the Present Invention>

The reason behind the exhibition of the effect of the present inventionis not yet very clear, but is presumed as follows.

For increasing the capacity of a battery for an automobile, use of alithium transition metal compound with a high capacity as a positiveelectrode active material has been tried. Such a lithium transitionmetal compound with a high capacity can be obtained, for example, bydecreasing the amount of Mn and increasing the amount of Ni.

However, it has been found with respect to such a positive electrodethat, when a nonaqueous electrolyte secondary battery is assembled,there arises a problem, namely the capacity retention rate after hightemperature storage is low, the amount of gas from storage and theamount of metal dissolution after high temperature storage increase, theresistance after high temperature storage is high, and the amount ofheat generation at a high temperature is large.

The inventors investigated this problem and found the followingpresumable mechanism.

That is, when the amount of Mn is decreased and the amount of Ni isincreased, the capacity is increased, and as a result the amount of Liin a crystal of the lithium transition metal compound at the time ofcharging is decreased. In this case, the oxygen atom becomes unstableand activated in the crystal of the lithium transition metal compound.For this reason, it is conceivable that the oxidation power of thepositive electrode increases, and the crystal structure at the surfacecollapses and changes to a rock-salt structure. It has been presumedthat the lithium transition metal compound with increased oxidizingpower decomposes a nonaqueous electrolyte solution in the battery tocause gas generation and metal dissolution, and to increase the amountof heat generation at a high temperature. It has been also presumed thatthe collapse of the crystal structure at the surface of the lithiumtransition metal compound causes decrease in the capacity and increasein the resistance after high temperature storage.

In this regard, the inventors have found that by increasing the platedensity of the positive electrode to a certain level or higher, andadding a specific phosphorus compound to the nonaqueous electrolytesolution, the destabilization of oxygen atoms in the crystal of thelithium transition metal compound may be suppressed, decrease in thecapacity retention rate after high temperature storage, or the amount ofgas generated by decomposition of a nonaqueous solvent of the nonaqueouselectrolyte solution and the amount of metal dissolution may besuppressed, the resistance after high temperature storage may bedecreased, and the amount of heat generation at high temperature may bedecreased.

Although the reason for this is also not yet very clear, it is presumedthat the specific phosphorus compound of the present invention becomeseventually LiF, when decomposed, and when this LiF appears in thesurface of the lithium transition metal compound, it reacts with thesurface of the lithium transition metal compound having increased theoxidizing power, and the destabilization of oxygen atoms in the crystalmay be suppressed, decrease in the capacity retention rate after hightemperature storage, the gas generation by decomposition of thenonaqueous electrolyte solution and the metal dissolution may besuppressed, the resistance after high temperature storage may bedecreased, and the amount of heat generation at high temperature may bedecreased.

EXAMPLES

Next, specific embodiments of the present invention will be described inmore detail by way of Examples, provided that the present invention isnot limited to the examples.

An abbreviation of the compound used in the examples is shown below.

Compound 1: Lithium Difluorophosphate

[Evaluation of Nonaqueous Electrolyte Secondary Battery]

Initial Charging and Discharging

In a constant temperature chamber at 25° C., a sheet-formed nonaqueouselectrolyte secondary battery was charged at 0.05 C (the current valueat which the rated capacity based on the 1-hour rate discharge capacityis discharged in 1 hour is defined as 1 C. The same applies hereinafter)for 4 hours, and then discharged at a constant current of 0.2 C to 2.5V. Next, 2 cycles of a charge and discharge cycle consisting of constantcurrent-constant voltage charging at 0.2 C up to a predeterminedvoltage, and constant current discharging at 0.2 C to 2.5 V, wererepeated.

Furthermore, after constant current-constant voltage charging at 0.2 Cup to 4.0 V, the battery was stabilized by storing it at 45° C. for 42hours. Thereafter, the battery was discharged at constant current at 25°C. to 2.5 V, and then constant current-constant voltage charging wasperformed at 0.2 C up to a predetermined voltage. Thereafter, constantcurrent discharge at 0.2 C was performed to 2.5 V, and the thendischarge capacity was defined as the capacity (A) before storage. Next,constant current-constant voltage charging was performed at 0.2 C up toa predetermined voltage. The predetermined voltage is usually 4.2 V, andmay be 4.3 V or 4.4 V.

Storage Test

The cell after initial charging and discharging was stored at a hightemperature under conditions of 85° C. and 24 hours. After the batterywas sufficiently cooled, it was immersed in an ethanol bath, and itsvolume was measured. The amount of gas generation was determined fromthe volume change before and after the storage test, and this amount wasdefined as the amount of gas from storage. The reduction rate of theamount of gas from storage by an additive was defined as a “suppressionrate for gas from storage” (for example, Suppression rate for gas fromstorage (%) of Example 1=[(Amount of gas from storage of ComparativeExample 1—Amount of gas from storage of Example 1)/Amount of gas fromstorage of Comparative Example 1]×100). The discharge capacity when thiscell was subjected to constant current discharge at 0.2 C to 2.5 V in aconstant temperature chamber at 25° C. was defined as the post-storagecapacity (B). The ratio of the post-storage capacity (B) to thepre-storage capacity (A) was defined as “capacity retention rate afterstorage”.

In this regard, it is preferable that the capacity retention rate afterstorage is as high as possible, and that the amount of gas from storageis as low as possible, because swelling of the battery can beinsignificant.

Resistance after Storage Test

After the storage test, the battery was charged at constantcurrent-constant voltage at 0.2 C up to 4.2 V, and then the resistancewas measured by applying an AC voltage (0.1 Hz) at 10 mV. This wasdefined as “post-storage resistance”, and the rate of resistancereduction by an additive was defined as “resistance suppression rate”.

Amount of Metal Dissolution after Storage Test

The amount of metal dissolution was determined by analyzingquantitatively the amount of metal deposited on the negative electrode.The amount of metal deposited on the negative electrode was analyzed byICP (high frequency inductively coupled plasma) emission spectroscopyafter acid decomposition of the negative electrode to determine thetotal amount of metal dissolution of Ni, Mn and Co, and the reductionamount of metal dissolution by an additive was defined as “metaldissolution suppression amount”.

Method for Measuring Amount of Heat Generation

A cell after initial charging and discharging was charged at constantcurrent-constant voltage at 0.2 C up to 4.5 V, from which the positiveelectrode was taken out and placed into a measuring cell together withan electrolyte solution and subjected to a measurement with a Calvetcalorimeter. The measurement was carried out by raising the temperatureto 300° C. at 1 K/min, and the ratio of the total amount of heatgeneration from 100° C. and 300° C. to the charge capacity up to 4.5 Vwas defined as “amount of heat generation per unit capacity”. Also, therate of reduction of the amount of heat generation by an additive wasdefined as “heat generation suppression rate”.

Method for Measuring Tap Density

For measuring the tap density of a lithium transition metal compound, asample was dropped in a 10 mL graduated cylinder to fill its volume,followed by tapping 200 times, and the density was calculated from thethen volume and the sample mass.

Method for Measuring Sulfate Salt and Carbonate Salt

A sulfate salt and a carbonate salt contained in a positive electrodeactive material were measured by water extraction ion chromatography.

Method for Measuring pH

For pH, 50 g of desalted water was weighed into a beaker, to which 5 gof a sample was added with stirring and the stirring was continued at25° C. for 30 min. Thereafter, the pH was measured while maintaining theliquid temperature at 25° C.

Method for Measuring Ni Valence

The valence was calculated from the analysis of the ratio of Li to eachtransition metal by water extraction ion chromatography, and byinductively coupled plasma atomic emission spectrophotometry (ICP-AES).

Example

[Production of Nonaqueous Electrolyte Secondary Battery]

<Preparation of Nonaqueous Electrolyte Solution>

Thoroughly dried LiPF₆ was dissolved in a mixture of ethylene carbonateand ethyl methyl carbonate (volume ratio 3:7) to 1 mol/L (as aconcentration in the nonaqueous electrolyte solution) in a dry argonatmosphere, in which thoroughly dried vinylene carbonate was dissolvedto 2% by mass to prepare a reference nonaqueous electrolyte solution.Then, in a case the compound 1 was further dissolved therein to 1% bymass (as the concentration in the nonaqueous electrolyte solution), andin the other case the compound 1 was not added, so that totally twokinds of nonaqueous electrolyte solutions were prepared.

A nonaqueous electrolyte secondary battery was produced by the followingmethod using either of the nonaqueous electrolyte solutions, and thefollowing evaluations were performed.

<Production of Positive Electrode>

As a lithium nickel manganese cobalt composite oxide as a positiveelectrode active material, one of the following 3 kinds was used:

NMC622 (Li_(1.00)Ni_(0.61)Mn_(0.19)Co_(0.20)O₂: molar ratio ofMn/(Ni+Mn+Co)=0.19, molar ratio of Ni/(Ni+Mn+Co)=0.61, sulfate saltconcentration=38 μmol/g, carbonate salt concentration 91 μmol/g, Niaverage valence=2.63, pH of aqueous solution=11.88, tap density=2.39g/cm³),NMC532 (Li_(1.05) Ni_(0.52)Mn_(0.29)Co_(0.20)O₂: molar ratio ofMn/(Ni+Mn+Co)=0.29, molar ratio of Ni/(Ni+Mn+Co)=0.52, sulfate saltconcentration=30 μmol/g, carbonate salt concentration=16 μmol/g, Niaverage valence=2.51, pH of aqueous solution=11.75, tap density=2.39g/m³), orNMC111 (Li_(1.05) Ni_(0.34)Mn_(0.33)Co_(0.33)O₂: molar ratio ofMn/(Ni+Mn+Co)=0.33, molar ratio of Ni/(Ni+Mn+Co)=0.34, sulfate saltconcentration=14 μmol/g, carbonate salt concentration=12 μmol/g, Niaverage valence=2.15, pH of aqueous solution=11.12, tap density=1.55g/m³).

In N-methyl-2-pyrrolidon, 94 parts by mass of each positive electrodeactive material, 3 parts by mass of acetylene black as a conductivematerial, 3 parts by mass of poly(vinylidene fluoride) (PVdF) as abinder, and 0.07 part by mass of polyvinylpyrrolidone as a dispersingagent were mixed and slurried. This slurry was uniformly applied to a 15μm-thick aluminum foil, dried, and roll-pressed to yield a positiveelectrode (this positive electrode is hereinafter sometimes denoted aspositive electrode 1). The plate density of the positive electrode wasregulated by changing the pressure of the roll press between 0 and 13kN/cm to prepare 5 kinds of positive electrodes having the respectivepositive electrode densities of 2.4, 2.8, 3.0, 3.2, and 3.3 g/cm³.

<Production of Negative Electrode>

To 49 parts by mass of graphite powder, 50 parts by mass of an aqueousdispersion of carboxymethylcellulose (1% by mass concentration of sodiumcarboxymethylcellulose) as a thickener, and 1 part by mass of an aqueousdispersion of styrene-butadiene rubber (49% by mass concentration ofstyrene-butadiene rubber) as a binder were added, and the mixture wasmixed with a disperser to form a slurry. The obtained slurry wasuniformly applied to a 10 μm-thick copper foil, dried, and roll-pressedto yield a negative electrode.

<Production of Nonaqueous Electrolyte Secondary Battery>

The aforedescribed positive electrode, negative electrode, andpolyolefin separator were layered one on another in the order ofnegative electrode, separator, and positive electrode. The batteryelement thus obtained was wrapped with an aluminum laminate film, filledwith the aforedescribed nonaqueous electrolyte solution, and then sealedin vacuum to form a nonaqueous electrolyte secondary battery in a sheetform. According to combinations of 3 kinds of positive electrode activematerials, 5 kinds of positive electrode plate densities, and 2 kinds ofnonaqueous electrolyte solutions with or without the compound 1 as setforth in Table 1, nonaqueous electrolyte secondary batteries of Examples1 to 4, and Comparative Examples 1 to 15 were produced.

TABLE 1 Positive electrode Additive to Positive electrode plate densityelectrolytic active material (g/cm³) solution Example 1 NMC622 3.3Compound 1 Example 2 NMC532 3.3 Compound 1 Example 3 NMC622 3.2 Compound1 Example 4 NMC622 3.0 Compound 1 Comparative NMC622 3.3 — Example 1Comparative NMC532 3.3 — Example 2 Comparative NMC622 3.2 — Example 3Comparative NMC622 3.0 — Example 4 Comparative NMC622 2.8 Compound 1Example 5 Comparative NMC622 2.4 Compound 1 Example 6 Comparative NMC6222.8 — Example 7 Comparative NMC622 2.4 — Example 8 Comparative NMC1113.3 Compound 1 Example 9 Comparative NMC111 3.2 Compound 1 Example 10Comparative NMC111 3.0 Compound 1 Example 11 Comparative NMC111 2.8Compound 1 Example 12 Comparative NMC111 3.3 — Example 13 ComparativeNMC111 3.2 — Example 14 Comparative NMC111 3.0 — Example 15

Table 2 shows the capacity retention rate after storage and thesuppression rate for gas from storage. As obvious from Table 2, thecapacity retention rate after storage is improved and the amount of thegas from storage is suppressed, when the compound 1 is added to anonaqueous electrolyte solution of a nonaqueous electrolyte secondarybattery provided with a positive electrode containing a positiveelectrode active material with a specific composition, and having aspecific plate density. In other words, a nonaqueous electrolytesecondary battery excellent in high temperature life is obtained. InComparative Examples 6 and 8, the capacity retention rate was asextremely low as 0.0% with 2 significant digits.

TABLE 2 Positive Positive Capacity Suppression electrode electrodeAdditive to Pre-storage retention rate rate for gas active plate densityelectrolytic capacity after storage from storage material (g/cm³)solution (mAh/g) (%) (%) Example 1 NMC622 3.3 Compound 1 173.9 95.7 69Example 2 NMC532 168.6 95.8 51 Comparative NMC622 — 173.1 95.2 — Example1 Comparative NMC532 167.6 95.1 Example 2 Comparative NMC622 2.4Compound 1 162.4 0.0 55 Example 6 Comparative NMC622 — 161.7 0.0 —Example 8 Comparative NMC111 3.3 Compound 1 156.4 93.3 35 Example 9Comparative NMC111 — 155.9 94.0 — Example 13

Table 3 shows the post-storage resistance and the resistance suppressionrate after storage. As obvious from Table 3, the post-storage resistanceis suppressed low, when the compound 1 is added to a nonaqueouselectrolyte solution of a nonaqueous electrolyte secondary batteryprovided with a positive electrode containing a positive electrodeactive material with a specific composition, and having a specific platedensity. Since the resistance inside the battery correlates with theamount of heat generation in the battery at the time of charging, anonaqueous electrolyte secondary battery, which amount of heatgeneration at the time of charging is little even after high temperaturestorage, and which is therefore superior in safety, is obtained.

TABLE 3 Positive Positive Resistance electrode electrode Additive toPost-storage suppression active plate density electrolytic resistancerate material (g/cm³) solution (Ω) (%) Example 3 NMC622 3.2 Compound 13.7 47 Example 4 3.0 4.2 46 Comparative 2.8 6.0 47 Example 5 Comparative2.4 20.2 93 Example 6 Comparative 3.2 — 6.9 — Example 3 Comparative 3.07.8 Example 4 Comparative 2.8 11.3 Example 7 Comparative 2.4 283.9Example 8 Comparative NMC111 3.2 Compound 1 4.0  2 Example 10Comparative 3.0 6.9 −6 Example 11 Comparative 3.2 — 4.1 — Example 14Comparative 3.0 6.5 Example 15

Table 4 shows the amount of heat generation per unit capacity, and heatgeneration suppression rate. As is obvious from Table 4, the amount ofheat generation per unit capacity may be suppressed low, when thecompound 1 is added to a nonaqueous electrolyte solution of a nonaqueouselectrolyte secondary battery provided with a positive electrodecontaining a positive electrode active material with a specificcomposition, and having a specific plate density. In other words, anonaqueous electrolyte secondary battery, which has a high capacity,generates only a small amount of heat, and is therefore safe, has beenobtained.

TABLE 4 Positive Positive Amount of Heat electrode electrode Additive toheat generation generation active plate density electrolytic per unitcapacity suppression rate material (g/cm³) solution (J/mAh) (%) Example1 NMC622 3.3 Compound 1 9.8 11 Comparative NMC622 2.8 9.9 2 Example 5Comparative NMC111 3.3 13.2 −11 Example 9 Comparative NMC622 3.3 — 11.1— Example 1 Comparative NMC622 2.8 10.1 Example 7 Comparative NMC111 3.311.9 Example 13

Table 5 shows the metal dissolution suppression amount. As obvious fromTable 5, the metal dissolution suppression effect is strengthened, whenthe compound 1 is added to a nonaqueous electrolyte solution of anonaqueous electrolyte secondary battery provided with a positiveelectrode containing a positive electrode active material with aspecific composition, and having a specific plate density. In otherwords, a nonaqueous electrolyte secondary battery, in which metaldissolution from the positive electrode is insignificant, and which istherefore safe and has a high capacity, has been obtained.

TABLE 5 Positive Positive Metal dissolution electrode electrode Additiveto suppression active plate density electrolytic amount material (g/cm³)solution (μmol) Example 1 NMC622 3.3 Compound 1 0.36 Comparative NMC6222.8 0.34 Example 5 Comparative NMC111 3.3 0.17 Example 9 ComparativeNMC111 2.8 0.04 Example 12

Example 5

[Production of Nonaqueous Electrolyte Secondary Battery]

<Preparation of Nonaqueous Electrolyte Solution>

Thoroughly dried LiPF₆ was dissolved in a mixture of ethylene carbonateand ethyl methyl carbonate (volume ratio 3:7) to 1 mol/L (as aconcentration in the nonaqueous electrolyte solution) in a dry argonatmosphere, in which 2% by mass of thoroughly dried vinylene carbonateand 1% by mass of the compound 1 (as the concentration in the nonaqueouselectrolyte solution) were further dissolved to prepare a nonaqueouselectrolyte solutions.

A nonaqueous electrolyte secondary battery was produced by the followingmethod using the nonaqueous electrolyte solution, and the followingevaluations were performed.

<Production of Positive Electrode>

As a positive electrode active material, 90 parts by mass of a lithiumnickel manganese cobalt composite oxide(Li_(1.00)Ni_(0.61)Mn_(0.19)Co_(0.20)O₂: molar ratio ofMn/(Ni+Mn+Co)=0.19, molar ratio of Ni/(Ni+Mn+Co)=0.61, sulfate saltconcentration=38 μmol/g, carbonate salt concentration 91 μmol/g, Niaverage valence=2.63, pH of aqueous solution=11.88, tap density=2.39g/cm³), as a conductive material 7 parts by mass of acetylene black, asa binder 3 parts by mass of poly(vinylidene fluoride) (PVdF), and as adispersing agent 0.07 part by mass of polyvinylpyrrolidone were mixed inN-methyl-2-pyrrolidon, and slurried. This slurry was uniformly appliedto a 15 μm-thick aluminum foil, dried, and roll-pressed to yield apositive electrode (this positive electrode is hereinafter sometimesdenoted as positive electrode 6). The plate density of the positiveelectrode 6 was 3.3 g/cm³.

<Production of Negative Electrode>

The respective negative electrodes of Examples 5 and 6, and ComparativeExample 16 were produced in the same manner as in Examples 1 to 4, andComparative Examples 1 to 15.

<Production of Nonaqueous Electrolyte Secondary Battery>

The respective nonaqueous electrolyte secondary batteries in a sheetform of Examples 5 and 6, and Comparative Example 16 were produced inthe same manner as in Examples 1 to 4, and Comparative Examples 1 to 15.

Comparative Example 16

A nonaqueous electrolyte secondary battery was produced in the samemanner as in Example 5 except that Compound 1 was not added in thenonaqueous electrolyte solution.

The capacity retention rate after storage, the amount of gas fromstorage, and the amount of metal dissolution after the storage test ofExample 6 and Comparative Example 16 are shown in Table 6.

Example 6

A positive electrode (this positive electrode is hereinafter sometimesdenoted as positive electrode 7) was produced in the same manner as inExample 5, except that the lithium nickel manganese cobalt compositeoxide (Li_(1.05)Ni_(0.52)Mn_(0.29)Co_(0.20)O₂: molar ratio ofMn/(Ni+Mn+Co)=0.29, molar ratio of Ni/(Ni+Mn+Co)=0.52, sulfate saltconcentration=30 μmol/g, carbonate salt concentration=16 μmol/g, Niaverage valence=2.51, pH of aqueous solution=11.75, tap density=2.39g/cm³) was used as a positive electrode active material, and anonaqueous electrolyte secondary battery was produced in the same manneras in Example 5, except that the above positive electrode was used. Inthis regard, the plate density of the positive electrode 7 was 3.3g/cm³.

Comparative Example 17

A nonaqueous electrolyte secondary battery was produced in the samemanner as in Comparative Example 16 except that positive electrode 7used in Example 6 above was used.

The capacity retention rate after storage, the suppression rate for gasfrom storage, and the amount of metal dissolution after the storage testof Example 6 and Comparative Example 17 are shown in Table 7.

TABLE 6 Capacity Suppression Amount of metal Pre-storage retention raterate for gas dissolution Electrolyte Voltage capacity after storage fromstorage after storage solution Additive (V) (mAh/g) (%) (%) (μmol)Example 5 1 mol/L Compound 1 4.2 173.9 95.7 69 — LiPF₆ 4.3 187.1 94.7 55— EC/EMC = 4.4 197.3 92.7 21 2.3 Comparative 3/7 + 2% — 4.2 173.1 95.2 —— Example 16 VC 4.3 186.3 94.1 — — 4.4 197.3 92.2 — 4.8

TABLE 7 Capacity Suppression Amount of metal Pre-storage retention raterate for gas dissolution Electrolyte Voltage capacity after storage fromstorage after storage solution Additive (V) (mAh/g) (%) (%) (μmol)Example 6 1 mol/L Compound 1 4.2 168.6 95.8 51 — LiPF₆ 4.3 180.5 95.0 29— EC/EMC = 4.4 191.2 93.3 3 2.0 Comparative 3/7 + 2% — 4.2 167.6 95.1 —— Example 17 VC 4.3 180.1 94.1 — — 4.4 190.4 91.9 — 5.5

As obvious from Table 6, the capacity retention rate after storage isenhanced, the gas amount from storage is suppressed, and the amount ofmetal dissolution is reduced, when the compound 1 is added to anonaqueous electrolyte solution of a nonaqueous electrolyte secondarybattery provided with a positive electrode having a specific platedensity and containing a positive electrode active material which has aspecific composition, and contains a specific amount of a sulfate salt.In other words, a nonaqueous electrolyte secondary battery, which isexcellent in high temperature life, and in which the amount of heatgeneration at a high temperature is small, has been obtained.

As obvious from Table 7, the capacity retention rate after storage isenhanced, the gas amount from storage is suppressed, and the amount ofmetal dissolution is reduced, when the compound 1 is added to anonaqueous electrolyte solution of a nonaqueous electrolyte secondarybattery provided with a positive electrode containing a sulfate salt. Inother words, a nonaqueous electrolyte secondary battery, which isexcellent in high temperature life, and in which the amount of heatgeneration at a high temperature is small, has been obtained.

The invention claimed is:
 1. A nonaqueous electrolyte secondary batterycomprising: a positive electrode with a positive electrode activematerial capable of absorbing and releasing a metal ion; a negativeelectrode with a negative electrode active material capable of absorbingand releasing a metal ion; and a nonaqueous electrolyte solution;wherein the positive electrode active material comprises a lithiumtransition metal compound, and the positive electrode active materialcomprises at least Ni, Mn and Co, wherein the molar ratio ofMn/(Ni+Mn+Co) is larger than 0 and 0.32 or less, the molar ratio ofNi/(Ni+Mn+Co) is 0.55 or more and 0.95 or less, the plate density of thepositive electrode is 3.0 g/cm³ or more; the nonaqueous electrolytesolution comprises a monofluorophosphate and/or a difluorophosphate, anda total content of the monofluorophosphate and/or difluorophosphate is0.01% by mass or more in terms of the concentration in the nonaqueouselectrolyte solution.
 2. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein the positive electrode active materialcomprises a lithium transition metal compound represented by thefollowing Formula (I):Li_(1+x)MO₂  (I) wherein, x is from −0.05 to 0.06, and M comprises atleast Ni, Mn and Co.
 3. The nonaqueous electrolyte secondary batteryaccording to claim 2, wherein the x is 0.028 or less.
 4. The nonaqueouselectrolyte secondary battery according to claim 1, wherein the molarratio of Mn/(Ni+Mn+Co) is 0.28 or less.
 5. The nonaqueous electrolytesecondary battery according to claim 1, wherein the molar ratio ofNi/(Ni+Mn+Co) is 0.55 or more.
 6. The nonaqueous electrolyte secondarybattery according to claim 1, wherein the plate density of the positiveelectrode is 3.2 g/cm³ or more.
 7. The nonaqueous electrolyte secondarybattery according to claim 1, wherein the positive electrode activematerial further contains a sulfate salt.
 8. The nonaqueous electrolytesecondary battery according to claim 7, wherein the amount of thesulfate salt contained in the positive electrode active material is 15μmol/g or more.
 9. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein an average Ni valence of the lithiumtransition metal compound is 2.1 or more and 3 or less in an unchargedstate.
 10. The nonaqueous electrolyte secondary battery according toclaim 1, wherein the pH of an aqueous solution of the lithium transitionmetal compound is 11 or higher based on a liquid temperature of 25° C.11. The nonaqueous electrolyte secondary battery according to claim 1,wherein the positive electrode active material contains a carbonate saltat 10 μmol/g or more.
 12. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein a tap density of the lithium transitionmetal compound is 1.8 g/cm³ or more.